So the day they make your particular fetish or recreational substance / entertainment / political stance / religion / etc. a crime...
You're an optimist, and overstating the safety and benevolence of this program. Quit sugar-coating it, you apologist!
Peoples' fetish, substance, etc is already illegal, somewhere. And since no government (including UK) has shown itself to have the ability to store things securely (it's almost as though they employ people), it is reasonable to assume the data is (or eventually will be) globally available.
UK citizens aren't just making a decision to totally and completely trust trust their own government forever. They are also deciding that they already fully trust the Russian government, the Chinese government, the Saudi government, criminals, etc and that they will always be able to trust those parties.
UK is declaring that this is one big happy world without any adversarial relationships, and that "security" is a totally obsolete concept.