Nothing. As per usual this law hasn’t been thought out by the people in trying to inact it.
The law requires companies to introduce back doors into their systems. Most of the companies will officially just no longer offer their service in the UK.
There is an absolutely no provision under the current law to do anything about individual citizens or foreign nationals who circumvent (use a VPN) these rules. So basically you’re fine to just carry on as usual and pretend the law doesn’t exist.
What will the UK government do about that? Nothing, because the companies don’t (won’t) officially have any presence in the UK, so there isn’t much pressure that can be put on them, and I highly doubt other countries are going to accommodate them, as they are been ridiculous.
You might say that the government will ban VPNs but firstly that is problematic for companies, and secondly I don’t think they actually know VPN’s exist as a thing anyway.
Sooo it’s the UK government being dumb and pigheaded again. Yeah, that tracks.
Maybe they’re hoping a few companies cave and the rest don’t become a big deal for whatever use cases they don’t like, so they can act like the law worked. I’m not so sure that’s possible, though.