St Paul's There are 3 domes and two galleries on the exterior of them which have great views over the Thames, Westminster and city of London.
Stone Gallery (52m from the cathedral floor 367 steps)
Golden Gallery (85m from the cathedral floor 528 steps) Access to the Galleries Access to the Whispering Gallery (257 steps), Stone Gallery (additional 119 steps) and Golden Gallery (additional 152 steps from Stone) is via stair only.
We recommend that those with pre-existing medical conditions, mobility difficulties or concerns with heights and confined spaces do not attempt this climb.
A video fly through of the gallery levels and their views is available on the multimedia guides.
I'd add that at two points I had to crouch / bend to get through narrow low stair cases
(for reference I am 183cm).
Primrose Hill
Whilst not a tall building does offer a free panoramic view of London from the north.
If I recall correctly you can see the Gherkin, London eye and BT tower.
Parliament Hill Located at the south end of Hampstead Heath parliament hill offers a panoramic view of London from the tall buildings in Canary Wharf and the City and the BT Tower.
20
Fenchurch Street More commonly known as the walkie talkie has a garden on the top floors 34-37th (160M roof hight)
which is free to visit with a booking (you need to book more than a week in advance in summer for weekends)
The Shard
The View on the 68-72nd floors with prices starting at £25.95
Aqua Shard is a bar on the 31st floor
doesn't require a reservation.
They do have dress code which I can attest is strictly enforced and no under 18's after 6pm.
Tower 42 Has a champagne/wine bar on the 42nd floor and a restaurant on the 24th floor .
You usually need to book for the bar, but sometimes they'll let you in without booking if there's space.
New Zealand House
The New Zealand society often hosts events(for a small fee) in the penthouse of New Zealand House near Piccadilly Circus
it is the tallest building in the area and has amazing views of Westminster the London eye, horse guards parade and the backyard of number 10.
London has an open house Once a year (normally in September)
London has an open house where you can get access to many of London's iconic buildings including the Gherkin(at
least in previous years).
Key attractions such as the Gherkin are ticketed (due to demand, all sites are free so if someone is selling the ticket its probably fake.)
so you will need to book in advance.
There are many other awesome places that few people know about also on offer, such as old war offices in underground bunkers and unused tube stops.
The list changes yearly
so this is by no means a sure bet.
Elizabeth tower
If you're resident in the UK you can take a tour up Big Ben by contacting your local MP.
travel.stackexchange.com/users/449/Stuart
At 1,016ft, The Shard is the tallest building in Western Europe.
If you're planning a trip to London after February 2013, the 360° viewing gallery on the 68th-72nd floors should be open.
This panorama captures the northern half of the view at present, though it will change as some of the half dozen major skyscrapers under construction in the centre of the image will have been completed before it opens.
travel.stackexchange.com/users/1025/Colin Pickard
One option is the Tate Modern .
On the 5th floor is a small coffee shop or cafe, publicly accessible and free to gain access.
It has a great view looking north over the Thames towards St Paul's and Central London.
Another option that costs is the London Eye .
It takes 30 minutes to complete a revolution, and I recommend it both during the day and at night - it's spectacular at any time - although perhaps not when it's raining as the drops form on the glass then.
Prices as of 2011 start from £16.74 for adults .
travel.stackexchange.com/users/101/Mark Mayo
Alexandra palace has some nice views : Centrepoint tower which has a club/restaraunt at the top
: Views from there:
travel.stackexchange.com/users/1021/NimChimpsky
These are currently negative answers, obvious tall buildings you might expect to be the ones to go for, but which can be ruled out due to lack of known ways for members of the public to access them: 30
St Mary Axe , officially The Swiss Re Building , colloquially The Gherkin .
This building has a restaurant on the 39th floor called "Searcys 40|30 Gherkin" , but that restaurant and bar are private so you can't get in there either unless booked for a private event.
One Canada Square , sometimes incorrectly referred to as Canary Wharf .
This is the tallest building in the city but has no public viewing platform!
(I can't find a photo of the view from One Canada Square on Wikipedia)
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The tower of Westminster Cathedral (not to be confused with Westminster Abbey) is not as tall as the more modern buildings but it offers nice views over central and west London.
travel.stackexchange.com/users/1317/Peter Hahndorf
A free place to visit to get stunning views is the Sky Garden near Fenchurch St!
There are restaurants up there, along with bars, and you can go outside.
The inside itself is also stunning, with a variety of plants and flowers to keep you entertained for just under an hour. http://skygarden.london/
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Stave Hill in Russia Dock Woodland provides a 360° view including the Canary Wharf skyscrapers.
travel.stackexchange.com/users/26201/Timothy Baldwin
I'm looking for publicly accessible tall structures in London from where I can take photos of the city.  The Monument to the Great Fire of London has pretty good views, and I understand you can get good views from the dome in St Paul's Cathedral. Any other locations?
What publicly accessible tall structures to take photos from exist in London?
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<london><photography><vantage-points>
travel.stackexchange.com/questions/2798
travel.stackexchange.com/users/159/Robert Anton Reese
Good places to try include the three domes and two galleries of St Paul's, Stone Gallery, Golden Gallery, Whispering Gallery, the Gherkin, London Eye and BT tower, The View, Tower 42, New Zealand House, Elizabeth Tower, Alexandra Palace, the Tate Modern, the Sky Garden near Fenchurch Street, Stave Hill  in Russia Dock Woodland, Primrose Hill or Parliament Hill. If you're resident in the UK you can take a tour up  Big Ben  by contacting your local MP. There are many places in London offering amazing views. Building include St Paul's, The Shard and The View. For outside locations, on the other hand, there are Primrose Hill and Parliament Hill. The London Eye offers an amazing view, as does Big Ben if you are UK resident.
London has a large number of tall buildings, old and new, offering great views over London. They include St Paul's, The Shard and The View. Outside areas that provide a good view over London include Primrose Hill and Parliament Hill. UK resident's can see London from a tour up Big Ben. The London Eye offers a great view over London.