The National Museum of Computing sits on the Bletchley Park site in Milton Keynes, in Block H, the first purpose-built computer centre in the world. It holds the world's largest collection of working historic computers and follows computing from the secret wartime work of the 1940s, through the mainframes of the 1950s to 70s, to the personal computers of the 1980s and beyond.
Tip: Book online in advance, though walk-ups are welcome. Allow at least 2 hours, and check the Days Open page first, as it doesn't open every day.