Day 1: Westminster only
Palace exterior or the Guard, then the park to the Abbey and the clock, then stop. Do not cross the river, enter the Tower, enter a museum, or add Regent Street. The South Bank, shopping and nightlife are other days.
Day 2: two museums
Two wings of the British Museum, or Natural History plus the V&A. Not three museums and the Tower.
Day 3: one far axis
Greenwich, Kew, Hampton Court or Windsor — pick one. Do not add city nightlife after a far axis.