London
There are many accents traditionally associated with London. The most well known of the London accents long ago acquired the Cockney label from London's East End, which is heard both in London itself, and across the wider South East England region more generally. The accent of a 21st-century Londoner varies widely; what is becoming more and more common amongst the under-30s however is some fusion of Cockney with a whole array of ethnic accents, in particular Caribbean, which form an accent labelled Multicultural London English (MLE). The other widely heard and spoken accent is RP (Received Pronunciation) in various forms, which can often be heard in the media and many of other traditional professions and beyond, although this accent is not limited to London and South East England, and can also be heard selectively throughout the whole UK amongst certain social groupings. Since the turn of the century the Cockney dialect is less common in the East End and has 'migrated' east to Havering and the county of Essex.
English language in southern England
The traditional Portsmouth dialect (affectionately known as the Pompey dialect, and less favourably as Pompey Slang) is an oddity and somewhat an isolate. It stands well apart both structurally and phonetically from all other local English varieties around it. The traditional origin story is that the Dockyards attracted an influx of East Londoners at the turn of the 20th century and the accent is based in "Dockyard slang". Though structurally it is based around an East London model, it has retained a lot of traditional Hampshire features. Whilst it follows the consonant rules of Estuary English closely, the vowel sounds are quite different. The dialect is not a recent phenomenon, and can be heard in the speech of local people in their nineties. The surrounding accents geographically are quite different, and few contain as many of the "Estuary-isms" found in the Pompey dialect and some of those may actually originate from Portsmouth, rather than Estuary English.