Washington DC; the District and Capitol.net and Washington Post websites useful.
If you can't make that trip right now (and sadly Milton Keynes is 3638 miles from Washington DC - otherwise I'd be over there now!) - virtual tours are available
Senate (elements of this tour maintained by the house no longer work)
House of Representatives
There are a series of resources - including virtual tours and videos available on the C-SPAN "The Capitol" website.