Openreach would be obliged to provide fibre service at no extra cost (IMO).
I am sure I read an ISPReview article some time ago about someone who got caught with this exact issue and was expected to pay. But my age may be catching up with me 
Probably this one?
https://www.ispreview.co.uk/index.php/2022/06/openre...
At the moment that's essentially a database error: the user was unable to (re)order FTTC because the database said it was no longer available, when it should have been. In the end it was corrected.
In the long term, I cannot see Openreach being able to say "we're taking away your copper line (to save money), and if you want to continue, you have to pay us X thousand pounds".
If nothing else, the Universal Service Obligation is still a thing. I do expect they could say "we're taking away your copper service, and here's this 4G/5G/6G router to replace it" - although many people would not like it.