I really enjoyed my visit to the Courtauld Gallery in London, it's a little below the radar which is good actually. It's small and manageable, easy to get to, has a good varied selection of art, and then has a nice cafe for a pickmeup afterwards.
Just be aware that they have special exhibitions running, and you can't just walk up and get tix for those. They seem to sell our far in advance, but there is still plenty to see without that.