1. The entire UK economy is dependent on everyone having access to cryptography so good that even Russia can't break it, therefore it is of paramount national interest that everyone does have access to it.
2. Bad people will use the exact same tech to avoid detection while planning and committing major crimes that have the potential to escalate to the level of undermining the state.
The only way to prevent #2 is to enforce extremely luddite laws that mess up #1.
In reality things get much harder to guess at when pontificating from armchairs, as when you have to account for the fact that the government and police aren't actually trying to perfectly enforce any specific law — not even the ones we all grew up assuming they "surely must" fully enforce because they were the worst horrors in the newspapers — it stops being clear how much any given law is supposed to help nor how much harm it will cause.