Guess which WW2 activity used a very large number of relays, and was made by the GPO (who used relays heavily in telephony), and was subsequently discontinued by the GPO at wars end as things to do with encoding and decoding scaled back thus flooding the electronics market in the UK with relays...

Tommy Thomas (head of the ERCC in edinburgh where my dad wound up and I therefore lived) worked at Manchester on the Mark 1. He never talked about it and I never asked, subsequently. We both wound up in Australia at the CSIRO, where Radiophysics had been started by people in the radar space, and that bled into their interest in Computing. it's a small world. He was the head of the IT division and I was a lowly researcher, our paths didn't cross much. I wish I'd talked to him more, about this stuff and computer history.