Seriously. You. Must. Do. This.
The Victoria Tower is the tallest tower in the Palace of Westminster and it houses the Parliamentry Archives. It includes the fascinating hole in the floor looking down on the Sovereign's Entrance which they use to know when to raise to royal standard as the Queen steps off the coach and looking down through does make you feel a little seasick. Also it includes the Acts Room. Absolutely fascintating tour that lasts about half an hour or so.
You can book in advance to go and use the Archives search room, but tours of the Tower come up from time to time (usually on IanVisits). In the meantime there is the online tour to whet your appetite:
http://www.parliament.uk/visiting/online-tours/virtualtours/victoria-tower/ read more