If you are in London, a visit to the sky garden is a must, the address is 20 Fenchurch Street. The entrance is free, but you need to book your ticket online and a time slot at skygarden.london and it will give you full information
A lift will whisk you up to the thirty-fifth floor in seconds and there is a cafe and restaurant. The views are spectacular and you can see the shard opposite
It is a very short walk from Bank or Monument underground stations, which I think is the best way.