One of my favorite places I have been to so far in London is the Sky Garden. The Sky Garden is a public space that is located at the top of the Walkie Talkie Building in Central London. The Sky Garden is exactly what it seems: it is a public garden with seating and a restaurant all at the top of one of London's many skyscrapers. It is completely free to visit; the only catch is the fact that you have to make reservations for a time to go a few weeks in advance. The best part of the Sky Garden is the 360 degree view you get of the city. It is absolutely stunning and perfect for photos.
My friends and I booked our trip to the Sky Garden about two weeks in advance. We had booked our reservation for a Tuesday at 3pm; so we went right after we got out of school. It was a great experience, and something that was really fun to do during the afternoon. The building is covered with floor to ceiling windows; so there are 360 degree views of the city. It was really cool because you could see the entire Thames from up there. Additionally, there were huge lush green gardens in the center of the space. The photos of the gardens and the city really do speak for themselves. It was a very nice place to just relax and hangout with my friends all the while enjoying the city!
If you plan on visiting London I would highly recommend checking it out! It was awesome that the whole experience was free to the public as well! Something I have noticed about London is the fact that there is much more public spaces than in the US. Here there are tons of places where people can just hangout that don't cost any money--it is really nice!