While you do have to know what you are looking for, and walk the long way around from the larger Bletchley Park site, the National Museum of Computing is still (as of last year) there with volunteers showing off the workings of a bunch of rebuilt computers including the Colossus. I can't recommend a visit, with a guided tour, enough. And you might as well visit Bletchley itself while you are there, which is a different experience, but still worth while.