My younger sister really wanted to go on the London Eye, and I really wanted to go up the Shard, but at £20 per person it was getting pricey. Enter London Skygarden.
At the top level of the “Walkie Talkie” building there is an observation platform that is free to go up. You do however need to reserve your time in advance, and I would suggest about 2 weeks in advance or risk missing out.
There was a really lovely view of the London skyline from up there, and I think possibly nicer than the Shard observation platform would be, as you get a nice view of the Shard itself.