They weren't all destroyed, some were moved to what became GCHQ (UK's equivalent to the NSA)

See Influence and Fate: https://en.wikipedia.org/wiki/Colossus_computer

It's Tommy Flowers I always felt sorry for, after the war he wanted a loan to build a computer was refused on the grounds that what he wanted to build was impossible. He knew it was possible but couldn't disclose that due to secrecy laws