Up to the Sky!
The Sky Garden is a public space on the 35 top floor of 20 Fenchurch street, in the heart of the City. I wouldn’t call it a garden, as you might be disappointed if you go there expecting to visit a garden, but more a 360 degree spot with great view across the City of London. The entrance is free but you’ll need to book it on their website in advance. They do have a pricey restaurant with unanimous reviews advising you to avoid it. Go there for the view and for the wander around, that’s worth the sight (But remember not for their restaurant!)