Are you going to do a working line takeover? Assuming you’re not moving on the same exchange that you’re currently on, then you’ll need to do that or place an order for a new number/line.
Place the order and then, if it doesn’t come back with FTTC availability, cancel the order.
If WLT, then you’ll need their number to be able to do this anyway….
(And if you want to lay their fears at rest, if you do have their number and want to move to a new ISP, then any order you do place against their number will* result in an advice of transfer to them (“sorry you’re leaving” letter) and they’ll be able to cancel the order.)
*actually that’s a maybe. But if you’re able to place a fibre order then the service they’re on will be LLU MPF, which follows the AoT process right now.