Which UK buildings should people visit at least once?

I do enjoy architecture, whether it's classic or quirky, I'm always on the look out for interesting buildings to explore.

I've picked Lincoln cathedral. I was actually in Lincoln for football and we took in the city centre the day after as we were staying overnight, so our visit wasn't primarily aimed at sightseeing.

The sheer size of it was the initial thing that struck me, and then as I got closer the intricacies of the stone work drew me in.