Note that there are two museums on the site of Bletchley Park, with different opening hours -- the historic site, including the main house and the huts, and the separate National Museum of Computing, which has the Colossus replica, the WITCH, and a large collection of other British computing machinery and memorabilia. If your interest is in computing history, you'll probably want to see both -- which means timing your visit when they're both open, so be sure to check.