if you do go, my favorite exhibit at Bletchley is the Harwell Dekatron a.k.a WITCH. It's a really good machine for learning how computers work, both because its so early meaning it has simple architecture, but also for other reasons:
It uses base 10 instead of base 2 to store values in its memory, so doing the calculations in your head is a lot easier
It's memory is made from special tubes that have a orange glow at one of 10 positions, meaning you can see the contents of the computer's memory in its entirety just by looking at the machine.
Building off the previous point, you can see the cpu register, which is a single small piece of memory that stores the current data to be modified, which again with the lit-up memory means you can see what the computer is doing at any givenm time.
Finally, the computer has a debugging switch which can pause and step forwards through each instruction the computer is performing, as well as a more granular mode that lets you step through individual parts of an operation (for example each digit being added in a adding operation separately)
It's not always running but if you go I hope you get to see it :)
EDIT: oh and I suppose I should mention there is more than one museum at bletchley, there is the big one that focuses on ww2, but the museum of computing is tucked away to the side a bit.