No. The losing ISP has to provide you with a MAC within 5 days, they can no longer refuse for contractual reasons. This all changed in February. I suppose they can then pursue you for any balance they believe is due, but I don't know what the exact figure would be for it to be legal. For example, they are no longer paying BT for your local service rental, so they might not be able to recover that as it's not a loss.