The U.K. government is close to eroding encryption worldwide
The UK Government repeatedly fails to understand that there are no boarders on the internet, and it'd be impossible to impose any without the kind of extreme restrictions of a totalitarian regime.Any measures without broad international cooperation will push  vast number of people towards darker corners of the internet, which will not just end up completely undermining what they are trying to achieve, it will make the problems worse.Meta alone have the power to make this law a miserable failure.
People will want to use WhatsApp, the government themselves use it extensively.
If meta refuses there is very little they can do. Facebook can continue to operate without a single person on the ground in UK. It might harm their business in some ways but it's definitely doable.
The government might be able to force/convince Apple and Google to take it out their app stores in the UK but such regional restrictions are easily bypassed and WhatsApp is popular enough to make people try it. So that would then normalise the practices such as side loading / jail breaking and avoiding regional restrictions. Cyber criminals would be rubbing their hands at the opportunities this creates and I am sure the peodos and terrorists this is meant to be stopping will jump at the chance to get in on the act.