Well this is the issue that they dont realise. Its something that fundamentally cant be enforced, sure they can get the big players who want to trade in uk to subscribe to it like apple and google, but they wont be able to enforce it on foreign companies who publish to the app store, or individuals who just download and compile open source messaging apps. People who dont want to be spied on will simply move to a secure method and those who dont know/care will continue to use apple etc.
Again blocking VPNs wouldnt be something that is possible as an ISP has no knowledge of whether I am VPN'n to my office to work from home or to my azure VM which is running an end point overseas.
I really cant believe it has got this far the only people who will possibly be caught by this will surely be the lowly street dealers and such who we aren't supposed to be targeting with this.
Encryption is at a point now where its not unfeasible for a amateur computer programmer to use widely available tools to create an encrypted messaging app which a) cant be broken without decades of brute forcing b) destroys all evidence after its read and c) is operated and served from somewhere outside of goverments reach.