.....why else would all the major Parties reverse their views on unregulated immigration?

I am not aware that any party has "reversed its views". No party, to my knowledge, has ever had a policy of "unregulated immigration", therefore there can have been no reversal of views. Certainly, there has been a sharpening of the rhetoric, especially from the Tories. There is a glaringly simple reason for that. The Tories are being led by the nose by the racists of UKIP and their camp-followers. The Tories have no answer to their simplistic, populist and bigoted " policies" so they panic and try to talk ever tougher. But talk is all it is. Think of all the failed promises of politicians over the decades. The Tory promise of getting immigration down to the tens of thousands was the biggest lie, the egregiously biggest broken promise, OF ALL TIME. Not a principle in sight.