Sky probably doesn't ask for a MAC as it is to MPF.
As the others have said, it looks like either Sky hasn't enabled the exchange for FTTC, which is unlikely seeing as FTTC has been available there for at least a year, (assuming the OP is out of his minimum term), or they are full themselves on that exchange.
I'm currently migrating to Sky and have not been asked for a MAC. I even phoned up to offer it to them.
Migration happens on Thursday so will see if this is actually successful then.