The three South Ken museums, each one is just 5 mins walk from South Kensington Tube Station. A quick photo gallery, one picture outside and one inside for each museum. As I tell anyone who has the bad luck to visit the museums with me, "Don't just look at the exhibits, look at the building!"
Spoiler for big but beautiful pics:
1. Natural History:-
2. Science:-

(Yes, the whole long building, from the main street front and passing beyond the funny gold roof)
3. The "Victoria and Albert Museum" focuses on art and design:-