I think (and hope!) it would likely only get applied to the biggest services, and would be enforced by removal from the app stores.

Then, the logical next step for the government when this doesn’t work would be to allow this requirement at the OS level.

To answer seriously: unfortunately, the UK is one step ahead with the Online Safety Act. They’ve already given Ofcom the power to enforce client-side scanning. Ofcom themselves are deciding whether they want to use this power yet and this should happen sometime next year.