Education Unit
House of Commons
1 Derby Gate
London SW1A 2DG

Note that taking a tour is not the same thing as sitting in the Strangers' Gallery and attending debates, which requires special tickets. For more details about all the various options for visiting each House, have a browse through the official site for tours of Parliament. Non-U.K. residents can write to their U.K. embassy months in advance to secure a "card of introduction." However, it is often possible to get in simply by queueing up at the St. Stephen's entrance at 1 p.m. on a weekday.

For details, see the helpful webpage devoted to Visiting Parliament.