You must book in advance because they’re incredibly popular, huge queues, long waits, and capacity limits. The Sky Garden (Fenchurch Building) is huge, beautiful and absolutely packed with people. Many people consider it a London must visit.

https://en.wikipedia.org/wiki/20_Fenchurch_Street#Sky_garden

Ok that one looks legit :)

I was thinking more about his experience with e.g. the last one on the page, Roof Garden at The Post Building:

> I still don't understand why it's here nor why it's open daily, nor why they insist on Photo ID "and a full written name" before they'll let you up. However I didn't get the chance to test this out yesterday because when I arrived the roof terrace was "closed due to essential maintenance work", inconveniencing probably nobody but myself.