The Bank of England
Open Door when you can get tours around the inside of this august institution for free. My tour's guide announced himself as having worked for the Bank for 48 years - I thought we had some part time retiree. Until, that is, I discovered what amazing fund of knowledge he had, and how he could answer any question thrown at him. Turns out he is the museum curator, I think I got lucky. I was totally blown away buy the riches inside the building, an astonishing construction dating mostly from the 20's, although some elements are hundreds of years older. The craftsmanship displayed is quite extraordinary. I really recommend you visit, it is open this next weekend, too (3rd July). It is also fascinating to see that the gold bar that you can heft was worth £100,000 in 2005, £330,000 now.