Everyone says go early but honestly 3-5pm is the sweet spot at london museums. Morning tour groups from coaches have left, school kids are gone, you can actually see the rosetta stone without fighting for space.

British museum after 4pm on weekdays feels completely different. Tate modern stays open til 10pm fri/sat and is peaceful around 5pm. Staff are more relaxed and sometimes chat about art. Plus if you're not a morning person like me, you're not dragging yourself out of bed at 8am just to queue outside tottenham court road tube.