Thats what i'm interested in - what happens to the (up to ) 25% that are not covered by FTTP?
That's easy: Openreach continue to supply you with copper service (ADSL/FTTC) exactly as before. The copper stop-sell only applies to those properties where FTTP can be ordered.
Separately, your copper service will have to move to SOGEA or SOTAP by December 2025 as part of the PSTN switch-off. There will be a national stop-sell on analogue voice lines from September 2023, but this just means that if you order a voice service, you'll get ADSL or FTTC with a router and a port to plug your phone into. It can still be delivered over copper, if FTTP isn't available to you.