We don't have region/geo locked or infrastructure locked providers. If you have fibre to your home then any supplier in the UK that supports it can be your ISP.
Caveat: fibre is telco infrastructure and is not limited, cable can still be limited ie only virgin media on virgin cable infra
Not quite true. Openreach (BT) is regulated and required to let anyone offer service over their network.
Any other provider that lays their own fibre is free to offer only their own ISP service. I don't have access to FTTP via OpenReach, but can get it from one other provider (CommunityFibre).
If another fibre provider ever becomes dominant enough they might well also end up regulated more tightly, but that's not the case for the time being
Caveat: fibre is telco infrastructure and is not limited, cable can still be limited ie only virgin media on virgin cable infra