Several exchanges are on the stop sell list which is stopping copper by the end of 2025 (?) so presumably FTTP should be available by then?
Not necessarily.
You're also confusing 2 separate things.
The PSTN switch off is targeted by 2025.
This means a traditional dial tone call service should be gone by 2025.
There will still be areas with no full fibre coverage at all by this point.
Anyone wishing to use a landline for calls will need to use VOIP over FTTP or VOIP over ADSL/FTTC.
Then there's the copper stop sell you mention... that has no 2025 target.
The target for this varies by exchange and unlike the PSTN switch off it won't cover everyone.
Once an exchange reaches a target FTTP coverage (roughly 75%) they will start with a "no go back" policy. So anyone on FTTP will not be able to return to copper.
Then they will announce a "stop sell", giving roughly 12 months prior warning of the stop sell.
This will mean anyone with access to a full fibre service cannot purchase a copper service.
OpenReach have the intention of covering as many properties on these exchanges as possible with FTTP but they won't cover every property on every exchange that's part of these programs.
There will continue to be wayleave issues and properties that it isn't economically viable to roll out full fibre.