'Given the argument for undoing Brexit is that the EU is "just a trade agreement"' The mistake I see here is that you are saying "The argument" and "is" implyi

'Given the argument for undoing Brexit is that the EU is "just a trade agreement"' The mistake I see here is that you are saying "The argument" and "is" implying that there is a singular argument/factor/opinion in play. This kind of simplification is not the way forward.