I must recommend a visit to the Skygarden, London. It's at the top of 20 Fenchurch Street, also known as the walkie talkie building. You do need to book tickets online and print them out and you do need photo id before you will be allowed in. However, having booked tickets means that queues are minimal. The lift up to the top is very quick and ear popping! There is an external viewing platform on one side of the building, a bar for drinks and snacks (not cheap but you could guess that). We were lucky to have a beautiful day to visit and Gary took these photos.
Given the cost of going to the top of the Shard, this is a great alternative to get a perspective on London.