This caught me out when I was a teenager. Luckily, there was a chap in the Museum of Computing who was spending his Sunday working on the Colossus, and he was happy to let me in and show me around! I'll never forget that kindness, it was truly a fascinating trip.
I was there a few years ago and will echo the fact that the folks I interacted with at the museum of computing were truly world class fantastic. When someone loves what they do and where they are it really shows.
My boss at Experian took our team to Bletchley Park intending to go to the NMOC but not having checked that was actually open. Hilarious.
Fortunately the small Memorial to the Polish Cryptographers who are the reason this was worth attempting is outside so we could visit that, about a third of the team were Polish (we hired away some subcontractors from a Polish firm, and then we were in turn bought by Experian) so visiting this memorial was a key part of our intent.
Also IIRC at that time the Bombe was still not in TNMOC, and Colossus was in a separate building so you could see it, but the rest of TNMOC was closed.
I've also seen an even earlier incarnation of TNMOC when I was younger, just basically a room heaped with obsolete computer gear, with some nerds who know roughly what it is but aren't equipped to properly exhibit or explain it. I remember they'd set up some video game consoles, something like the original Sonic The Hedgehog maybe, because they wanted children to have some positive impression, obviously a kitchen appliance sized "disk drive" doesn't mean anything to a child, but it's obvious Sonic is a game even if you're used to much flashier graphics.
I found chatting to the people working on stuff was the best bit.
Almost the level of ignoring the exhibits and look at the bits of stuff being worked on around.
