Signal could block all UK IP addresses from accessing its messaging server. UK could require ISPs to block access to Signal messaging servers. Both of these are fairly trivial (my understanding is that UK already forces filtering on ISPs for e.g. pornography).
Of course you are correct that anyone motivated and/or technical enough could still access Signal. However, their less motivated/less technical contacts won't be using it any longer.
The government is basically asking the bad actors to self-identify through their continued use of Signal.
Why should Signal block UK IPs anymore so than China IPs, though? If anything, I hope that they have a page on the website specifically for UK users explaining the law and how to break it by installing the .apk directly.
Another reason why those arguing against centralized messaging systems as opposed to de-centralized, federated were correct. Who wants Signal and their US-centric board to decide they can do anything.
If Signal were really sticking it to the UK-vassal state they would be able to continue to provide end-to-end encryption and force their EU aircraft carrier to go through the public, international embarrasment of running their own version of the Great Firewall.
Of course you are correct that anyone motivated and/or technical enough could still access Signal. However, their less motivated/less technical contacts won't be using it any longer.
The government is basically asking the bad actors to self-identify through their continued use of Signal.