Made a quick visit to the Tate Museum today, which houses a collection of British-only art from 1500s-present. Admission is free to permanent galleries, and there are free guided tours available at different times of the day. The closest railway station is Pimlico, but we walked across the Thames from Vauxhall. Depending on how much you like British art, you could spend anywhere between 2 hrs to a half-day here. Mark & I were primarily interested in older artworks (pre-1900), so we finished in about 2.5 hrs.
Unfortunately, the one painting we both wanted to see, Ophelia, is currently on tour in Russia. 😦