As I've posted previously, the car makers would definitely move to the the EU mainland, the assemblers first, Toyota, Honda, PSA-Vauxhall, but Nissan would not be long after them and probably Jaguar-Land Rover.
In less than a decade we'd be back to the early 1970s when the IMF had to come in to rescued us from bankruptcy. The Leavers say this is project fear, but they'll soon really know what fear is if we do leave in this WTO way.
However, I don't see it happening, as daft as this government is they do know the facts of life.
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We are too closely interlinked now for that to happen now.
End of the day governments are there to help business not stop them. The Italian/French/Portuguese are responsible to their people first. Not the club. This is why everyone is watching so closely. A lot Europeans hope we do well so they can ask for better terns etc. The ruling class less so.
The Irish government will hurt their people for the chance of a petty point scoring exercise
But I predict nothing much will change even with a hard brexit. If Nissan decide to leave then we can approach the south koreans, even american car firms may invest. As @Zlatan says India are interested. They already have big investment here.
Change is not something to be scared of.