OP has apparently migrated his broadband and voice services to a Cuckoo FTTC 12 month contract - if he was to cease just the broadband (i.e. switch to FTTP broadband only during that contract period) I wonder if that would automatically terminate the voice service together with a possible loss of phone number also?

If/when FTTP becomes available I'll probably parallel run the FTTP broadband only alongside the FTTC broadband/phone line for couple of months as candlerb states. Then move the phone number to a VOIP provider and finally cancelling the FTTC.

Yes that's a good plan especially if FTTP eventually becomes available near the end of your contract with Cuckoo