Had a couple friends recently who had Openreach FTTP installed and in both cases Openreach have completely removed the copper lines regardless of existing VDSL service being in place (In At least one of those cases It was a new order NOT a migration)
If the VDSL service is live, and it's not a migration, then they can't do this. You should simply report a fault on the VDSL service via your CSP, and they'll have to reinstate the copper. If the CSP is a member of the OFCOM auto-compensation scheme then you should get paid a substantial amount per day for the amount of time it's out of service beyond 2 days.
Is this always the case for Openreach FTTP, does it make a difference if it's a business vs residential?
Normally, if the service is fed through an underground duct they'll just pull the fibre alongside it. If it's fed from overhead then they *can* replace the copper drop cable with a "hybrid" cable (fibre + copper), although it will depend on what supplies they have available. Or they could run two cables side by side, if the pole has sufficient loading capacity.
In my experience, they generally leave the copper in case it's required in future - although in the case of a migration and in a copper "stop sell" area, I guess they may not.
However, if you tell them on the day that the copper is live, then they should not disconnect a live service. If that means abandoning the appointment because they can't get the right cable from stores, then they can do that (and you'll also get compensation in that case).