I can see more issues with human error as there's no system process for FTTC > FTTP. I would hope BT Retail don't cease the FTTC service until the FTTP service is up and running.
To follow up, I finally managed to order FTTP after getting the run around a few times...
Openreach surveyed our property yesterday and I signed the permissions for them to dig on our property.
FTTC is still up-and-running, so as long as the duct laying doesn't wipe out our existing cooper line (and it shouldn't as we know the precise line it takes on our property as it was dug up during previous building works and reburied) we are looking good for a smooth transition. Phew!
Told to expect the duct laying within a couple of weeks, so might, just might, be good for xmas.
Current service date is 21st December, but this relies on the duct being in place in time...