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).
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.
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 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.
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.
If you're resident in the UK you can take a tour up Big Ben by contacting your local MP.