I'm no cat expert but they've always struck me as being particularly mercenary. Unlikely to stay somewhere where they aren't getting fed. I suspect also mostly incapable of looking after themselves long term. See the St Kilda situation but I'm aware there are some Greek islands with problems with unowned domestic cats. A small Greek touristic fishing village is probably an easier place to make a living as indigent cat than the Highlands.

In any event, Swinney sounded pretty unequivocal about the government not attempting a ban so I expect we'll find out whether a ban is actually practicable later this year.