From my underatanding, it's effective because All it has to do is force Operating Systems to scan data on personal devices before encryption. So, Microsoft (Windows), Apple (iOS & macOS) and Google (Android w/GMS) would comply.
Unless most people switch to small niche OS (e.g. niche Linux Distros or De-googled Android) that can't be forced to follow that law. 

 Well luckily the bill has been pulled because companies said the tech doesn’t exist to scan for illegal material without compromising privacy, and therefore threatened to remove products from the UK market.

I’d love to see how our current government would cope without WhatsApp 😂