…where does all this leave the Irish border?
Neither option, on its own, solves the Irish border conundrum, which is the greatest obstacle to a pro-free trade Brexit.
If the UK adopts a partial customs union, the state of the border between Turkey and EU member Bulgaria suggests there would still be some checks and delays, as on that crossing queues can stretch as long as four miles.
Full-on customs union membership doesn’t square the circle, either. It would spare the UK from costly rules of origin requirements, but checks to ensure goods meet EU regulatory standards would still have to be carried out.
This means physical infrastructure would appear on the border to police the new regime, which is a red line in the negotiations.
“You would still have an issue with the Irish border, but it’s less existential. Even a full customs union won’t solve this,” added Ms Renison.
“That said, most borders exist due to an absence of a customs union, so it is at least an important part of the puzzle.”