Excited today to discover we were driving past Bletchley Park, famous (in England at least) for Alan Turing and code breaking in the second world war (see The Imitation Game); so we went! The well known code breaking has the German Enigma machine as it’s target; of course such a machine is on display – it encrypts typed letters via a complex combination of wheels, and plug wiring boards – the wheels rotate before each letter encryption, so that the code constantly changes. We learned today that England entered the war with a lot of knowledge about the machine; but decryption initially took many weeks – so the results generally were not useful.
Alan Turing was convinced that machines could be used to speed up the process, and developed the “Bombe” that basically trials multiple wheel combinations against the encrypted text (received perhaps wirelessly), and a “guess” – hopefully a good guess! Eventually the machine was successful and sped up decrypts from weeks to hours, making them actually useful.
Bletchley Park has built a fully functional replica (difficult, since all original documentation was destroyed); we got to watch it work today:
More impressive perhaps is the Tunny machine that targeted Lorenz ciphers (used by German high command, so any results were important); this decryption machine incredibly was designed with no knowledge of the Lorenz machine – it was mathematically deduced from an erroneous repeat encryption!
A second museum at Bletchley Park has more “modern” computers, especially many from my formative years – I owned these three, the Acorn System One, Acorn Atom, and ZX Spectrum:
One last amazing thing there, is this huge storage disk platter – able to store a massive 4MB each side!
M&I / GBMINI 
Is that a BBC Micro in the background of the Acorn Atom picture? It wouldn’t surprise me as they were closely related I think.
There were BBC micros everywhere!