You can spend days in the British Museum, and photography is allowed everywhere.
"Street photography" is available everywhere in London. It's full of streets. If this is your first London visit, you'll find about a zillion people on Oxford Street. Take the Tube to Westminster Station, get out and gawk at Parliament and Big Ben. Then, walk across Westminster Bridge, turn left, and , shazam, you're on the South Bank. Keep walking by the crowds at the London Eye along the Thames and turn back when your feet hurt.
Take the Tube to Leicester Square. Go walkabout in Covent Garden, or in Chinatown, or Soho. Stop into Foyle's and buy a bunch of books.
That's two days.
London is a great place to combine tourism, walking, and photography.