Ok so everyone hits the british museum at opening time like it's black friday sales. Here's where to go when you're not a morning person:

Wellcome collection on euston road - weird medical history stuff, never crowded, opens 10am and still quiet at 2pm. Completely free and 5 mins from warren street tube.

Sir john soane's museum on lincoln's inn fields - tiny house crammed with random treasures. Gets busy for candlelit evenings but dead quiet weekday afternoons.

Horniman museum in forest hill - victorian taxidermy and surprisingly decent aquarium. Bit of a trek on the overground but worth it if you absolutely hate crowds.