I guess that anybody ceasing a VDSL service would not be allowed to obtain a new one in the future ?
Does anybody know if obtaining a MAC would be prohibited too ?
There shouldn't be any problem at all in obtaining a MAC. The difficultly could be in using it.
If the system says No, then the gaining ISP, would have big problems trying to place an order to enter the MAC key against.
Depending upon who the losing ISP is, you could ask for a MAC, and attempt a migration. If the gaining ISP says no, then you cancel the MAC. Some ISPs such as Plusnet dont close down the old account until the MAC key has been used. It could be a risky process though and there is a possibility that the line could lose vdsl.
Depends how hard the gaining ISP wants to fight for the EU.. and also what the system says for the line. If that was mine, I certainly wouldn't be risking a move to say TT/Sky, but I may consider somewhere like AAISP or Zen where they'd be more likely to fight for the line. I'd also be ringing them up first to explain the circumstances, so that they can check what the system says for that particular line and the ISP knows what they could potentially be dealing with.