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You're comparing the UK to the USA there and assuming the USA is saner.

The origin of the USA's prohibition against self-incrimination was people who were tortured into confessions and other forms of undermining their own freedom. The original logic for this right does not apply to divulging encryption keys. It's not clear what higher purpose is being served by allowing people under investigation to halt those investigations by using encryption: it's not like they can be tortured into revealing incorrect information inside the encrypted files.