London is of course an interesting tangle of buildings, especially the newer buildings in the City, like the Gherkin.
For older stuff, I would seriously recommend Bath. Much of it is well preserved from a couple centuries ago, much of it is made of the same limestone, called (I am not making this up) Bath Stone. The architecture is Georgian, symmetrical, and gorgeous.
I remember taking a bus from London to see Bath. We rounded a corner from a mountain, and suddenly Bath appeared--and the other passengers on the bus, mostly tourists, gasped when they saw how beautiful it was. My father, for reasons unknown to me, kept calling it "The Great Bath." This made me think of very large rubber duckies.