Tours of Congress

Tours of the US Capitol can be booked through the US Capitol Visitors Center. If that involves a trip to Washington you might find the 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.