You 'got it in one' in your last line SCO. The huge trading partner is the EU and the UK was stupid enough to break away as a US-EU trading member, one of many, and naively believe that they will be financially better off as an insular, separate entity - utter folly that will be proved in years to come as the United adjective in our UK acronym becomes fragmented in a divided country where it's clear that our population has been 'split down the middle' and polarised by such political folly of the highest order - it's even self-evident in parliament itself!