A lot of people have recommended visiting cathedrals, especially St. Paul's. And St. Paul's is extremely cool -- the Whispering Gallery really is amazing. Whichever you visit, though -- and I'd recommend Salisbury Cathedral, if you're going to get out into the country and see, say, Stonehenge -- see if you can find any of the literature (usually in the gift shop) about how people built such goram huge stuff nearly eight hundred years ago. The contraptions they built to haul huge blocks of stone high into the air are to die for, when it comes to "old-school tech".
Oh, yeah: If you're at all a music (or physics) geek, I think Salisbury has a 64' rank on its pipe organ. I might be making that up, though. Have a great trip.