In reply to Steve Perry:
> National Art Gallery in Trafalger Square is well worth a visit.
Second this.
Saw some amazing Rembrandt and Van Dyke masterpieces there, some of them are almost photographic, amazing skin tones. I also saw some Van Eyke, a huge leap forward in artistic skill in my opinion, I think the Arnolfini Portrait is there, quiet amazing, especially considering it's nearly 600 years old.
Of course, there's plenty of other stuff there that's jaw dropping to see "in the flesh". The Tate is great too, especially if Joeseph Turner floats your boat. In fact, you could spend a week in London just getting visual overlaod around the many great galleries.