Why do the courts need to differentiate VPN packets? They will just say "uhm, Mozilla, your public VPN product that is offered for purchase in the UK is not compliant with the Online Safety Act. Change it or we'll fine you one miillllion dol..pounds.".

Not saying I like it, but the idea that they can't do it is silly. And there are increasing levels of "China" that they can go to it they way:

1. Block access to foreign VPN websites.
2. Block VPN access to foreign VPN providers (trickier, but they could definitely degrade the experience).
3. Block all VPN connections at the ISP level except to whitelisted IPs. Companies can apply to the government to get their VPN IPs whitelisted.

Don't imagine that they can't become China for technical reasons. The only thing holding them back is politics and it isn't doing a good job of that.