Oh, forgot to mention, try and go to the Imperial War Museum. It's also free, and very impressive. Outside the front entrance they mounted a pair of 16" (or so) naval cannon which are themselves worthy of the trip. This is not to mention the P-51, Spitfire and FW-190 they have hanging from the ceiling in the main hall, right next to Snoopy's Sopwith Camel, and the Sherman tank, and the Panther tank, etc., etc. And yeah, take the tube.