The Barbican Conservatory is a peaceful oasis in the heart of the City of London. It takes a bit of effort to find all the way up on Level 3 of the main Barbican building, but it's definitely worth it! It was planted between 1980 and 1981, and opened in 1984, and there are now around 1,500 species of plants and trees from a mix of temperate and arid climates. The ponds contain koi and carp. It's free to get in but you need to book a ticket (during COVID times, at least). These are released a week in advance online on Thursday mornings. I was there before the Akari light sculptures by Isamu Noguchi went up: https://www.barbican.org.uk/whats-on/2021/event/visit-the-conservatory-akari-cloud
so I might need to go back to enjoy that magical mix of art and nature!