Westminster Hall
After visiting the two Houses, visitors pass back through St. Stephen's Hall, and through Westminster Hall. Westminster Hall is one of the few areas of Parliament in which photography is permitted, and it is a very impressive place, dating back to the 9th century. Plaques on the

After visiting the two Houses, visitors pass back through St. Stephen's Hall, and through Westminster Hall. Westminster Hall is one of the few areas of Parliament in which photography is permitted, and it is a very impressive place, dating back to the 9th century. Plaques on the
Check the official site before travelling; hours and prices can change.
More museums

Sir John Soane's Museum
Free, tiny, deranged — Soane's own house preserved exactly as he left it in 1837.

The Wallace Collection
Free national museum in a Manchester Square townhouse — armour, Boucher, the Laughing Cavalier.

The British Museum
Free, vast, civilisationally heavy — go for one room, not all of them.
Source: wikivoyage