Postal system in the United Kingdom
The General Post Office (GPO) was the state postal system and telecommunications carrier of the United Kingdom until 1969. Established in England in the 17th century, the GPO was a state monopoly covering the dispatch of items from a specific sender to a specific receiver (which was to be of great importance when new forms of communication were invented); it was overseen by a Government minister, the Postmaster General. Over time its remit was extended to Scotland and Ireland, and across parts of the British Empire. 
The GPO was abolished by the Post Office Act 1969, which transferred its assets to the Post Office, so changing it from a Department of State to a statutory corporation. Responsibility for telecommunications was given to Post Office Telecommunications, the successor of the GPO Telegraph and Telephones department. In 1980, the telecommunications and postal sides were split prior to British Telecommunications' conversion into a totally separate publicly owned corporation the following year as a result of the British Telecommunications Act 1981. In 1986 the Post Office Counters business was made functionally separate from Royal Mail Letters and Royal Mail Parcels (the latter being later rebranded as 'Parcelforce'). At the start of the 21st century the Post Office became a public limited company (initially called 'Consignia plc'), which was renamed 'Royal Mail Group plc' in 2002. In 2012 the counters business (known as 'Post Office Limited' since 2002) was taken out of Royal Mail Group, prior to the latter's privatisation in 2013. The privatised holding company (Royal Mail plc) was renamed International Distributions Services plc in 2022.
Early postal services.
In the medieval period, nobles generally employed messengers to deliver letters and other items on their behalf. In the 12th century a permanent body of messengers had been formed within the Royal Household of King Henry I, for the conveyance of royal and official correspondence. The messengers delivered their messages in person, each travelling on his own horse and taking time as needed for rest and refreshment (including stopping overnight if the length of journey required it). Under Edward IV, however, a more efficient system was put in place (albeit temporarily) to aid communications during his war with Scotland: a number of post houses were established at twenty-mile intervals along the Great North Road, between London and Berwick, to provide the king's messengers with fresh horses for each stage of the journey; in this way they were able to travel up to a hundred miles a day. 
Under King Henry VIII a concerted effort was made to maintain a regular postal system for the conveyance of royal and government despatches (in times of peace as well as in time of war). To oversee the system, the king appointed Brian Tuke to serve as 'Master of the Postes'. By the 1550s five post roads were in place, connecting London with Dover, Edinburgh, Holyhead (via Chester), Milford Haven (via Bristol) and Plymouth. (A sixth post road, to Norwich via Colchester, would be added in the early 17th century.) Each post-house on the road was staffed by a postmaster, whose main responsibility was to provide the horses; he would also provide a guide to accompany the messenger as far as the next post house (and then see to the return of the horses afterwards). In practice most post-houses were established at roadside inns and the innkeeper served as postmaster (in return for a small salary from the Crown). 
At Dover merchant ships were regularly employed to convey letters to and from the continent. A similar system connected Holyhead and Milford to Ireland (and by the end of the 16th century a packet service had been established on the Holyhead to Dublin route). Private citizens could make use of the post-horse network, if they could afford it (in 1583 they had to pay twopence per mile for the horse, plus fourpence per stage for the guide), but it was primarily designed for the relaying of state and royal correspondence, or for the conveyance from one place to another of individuals engaged on official state business, who paid a reduced rate (fixed by statute at a penny a mile in 1548). Private correspondence was often sent using common carriers at this time, or with others who regularly journeyed from place to place (such as travelling pedlars); towns often made use of local licensed carriers, who plied their trade using a horse and cart or waggon, while the universities, along with certain municipal and other corporations, maintained their own correspondence networks.
Many letters went by foot-post rather than on horseback. Footposts or runners were employed by many towns, cities and other communities, and had been for many years. A 16th-century footpost would cover around 30 miles per day, on average. At the time of the Spanish Armada every parish was by royal command required to provide a footpost, and every town a horse-post, to help convey news in the event of an imminent invasion. In Ireland the first horse-posts appear to have been established during the Nine Years' War at the close of the 16th century, for the conveying of military intelligence.
By the early 1600s there were two options for couriers using the post system: they could either ride 'through-post', carrying the correspondence the full distance; or they could use the 'post of the packet', whereby the letters were carried by the guides from one post house to the next in a cotton-lined leather bag (although this method was only available for royal, government or diplomatic correspondence). The guides at this time were provided with a post horn, which they had to sound at regular intervals or when encountering others on the road (other road users were expected to give way to the post riders).
Foreign postage.
At the start of the 16th century a system for the conveyance of foreign dispatches had been set up, organised by Flemish merchants in the City of London; but in 1558, after a dispute arose between Italian, Flemish and English merchants on the matter, the Master of the King's Posts was granted oversight of it instead. In 1619, James I appointed a separate Postmaster General 'for foreign parts', granting him (and his appointees) the sole privilege of carrying foreign correspondence to and from London. (The separate Postmaster General appointments were consolidated in 1637, but the 'foreign' and 'inland' postal services remained separate in terms of administration and accounting until the mid-19th century).
The General Post.
It was not until 1635 that a general or public post was properly established, for inland letters as for foreign ones. On 31 July that year, King Charles I issued a proclamation 'for the settling of the Letter-office of England and Scotland', an event which 'may properly be regarded as the origin of the British post-office'. By this decree, Thomas Witherings (who had been appointed 'Postmaster of England for foreign parts' three years earlier) was empowered to provide for the carriage of private letters at fixed rates 'betwixt London and all parts of His Majesty's dominions'. To this end, the royal proclamation instructed him to establish 'a running post, to run night and day', initially between London and Edinburgh, London and Holyhead and London and Plymouth, 'for the advancement of all His Majesty's subjects in their trade and correspondence'. (A similar system, running between London and Dover, had already been established by Witherings as part of his administration of the foreign posts, and he himself had proposed its extension to the rest of the realm). Witherings was required to extend the new system to other post roads 'as soon as possibly may be' (beginning with the routes to Oxford and Bristol, and to Colchester, Norwich and Yarmouth); and provision was also made for the establishment of 'bye-posts' to run to and from places not directly served by the post road system (such as Lincoln and Hull). The new system was fully and profitably running by 1636.
In order to facilitate the new arrangement, the King commanded 'all his postmasters, upon all the roads of England, to have ready in their stables one or two horses [...] to carry such messengers, with their portmantles, as shall be imployed in the said service', and they were forbidden from hiring out these horses to others on days when the mail was due. Furthermore, it was enjoined that (with a few specific exceptions) 'no other messenger or messengers, footpost or footposts, shall take up, carry, receive or deliver any letter or letters whatsoever, other than the messengers appointed by the said Thomas Witherings', thus establishing a monopoly, which (under the auspices of the Royal Mail) would remain in place until 2006. 
Under the Commonwealth the Post Office was farmed to John Manley and John Thurloe, successively. In 1657 an Act of Parliament entitled "Postage of England, Scotland and Ireland Settled" set up a postal system for the whole of the British Isles (the nations of which had been unified under Oliver Cromwell as a result of the Wars of the Three Kingdoms), stating that 'there shall be one General Post-Office, and one office stiled the Postmaster-Generall of England and Comptroller of the Post-Office'. The Act also reasserted the postal monopoly for letter delivery and for post-horses; and it set new rates both for carriage of letters and for 'riding post'. During the Commonwealth, what had been a weekly post service to and from London was increased to a thrice-weekly service: letters were despatched from the General Letter Office in London every Tuesday, Thursday and Saturday evening, while the inbound post arrived early in the morning on Mondays, Wednesdays and Fridays. Usually the recipient of the post paid the fee (and had the right to refuse to accept the item if they did not wish to pay); the charge was based on the distance the item had been carried so the Post Office had to keep a separate account for each item.
After the Restoration, the Post Office Act 1660 (12 Cha. 2. c. 35) was passed (the previous Cromwellian Act being void), confirming the arrangements in place for the Post Office, and the post of Postmaster General, and emphasizing the public and economic benefits of a General Post system: ;templatestyles src="Template:Blockquote/styles.css" / ;"Whereas for the maintenance of mutual correspondencies, and prevention of many inconveniences happening by private posts, several public post-offices have been heretofore erected for carrying and recarrying of letters by post to and from all parts and places within England, Scotland, and Ireland, and several posts beyond the seas, the well-ordering whereof is a matter of general concernment, and of great advantage, as well for the preservation of trade and commerce as otherwise".
To begin with the Post Office was again farmed, nominally to Henry Bishop, but the deal was bankrolled by John Wildman (a gentleman of dubious repute, who kept a tight rein on his investment). Two years later Wildman was imprisoned, implicated in a plot against the King, whereupon Bishop sold the lease on to the King's gunpowder manufacturer, Daniel O'Neill; after the latter's death, his widow the Countess of Chesterfield served out the remainder of the original seven-year term (so becoming the first female Postmaster General). Meanwhile, under the terms of a 1663 Act of Parliament, the 'rents, issues and profits' of the Post Office had been settled by the King on his brother, the Duke of York, to provide for his support and maintenance. Following the latter's accession to the throne as King James II, this income became part of the hereditary revenues of the Crown; and subsequently, under the growing scrutiny of HM Treasury, the postal service came increasingly to be viewed as a source of government income (as seen in the Post Office (Revenues) Act 1710, which increased postal charges and levied tax on the income in order to finance Britain's involvement in the War of the Spanish Succession).
Distribution and delivery.
The distribution network was centred on the General Letter Office in London (which was on Threadneedle Street prior to the Great Fire of London, after which it moved first to Bishopsgate and then to Lombard Street in 1678). The incoming post arrived each week on Mondays, Wednesdays and Fridays; it was sorted and stamped: London letters went to the 'windows' where members of the public were able to collect them from the office in person (once they