Not sure why Brexit would impact Britain's economy one way or the other, unless the EU is deliberately playing hardball, which is a good reason that Britain got out when they could. Simply said, there is no reason why Britain and the EU cannot have a bilateral trade agreement that provides all the economic benefits that EU membership did, nor is their reason not to enter into such an agreement.

Does Canada and Mexico have to come under US political control to enjoy the benefits of fair trade or does our current trade treaty provide that?

My sense is Britain's issues have more to do with the long-term effects of being a "nanny state" and the costs associated with that.