The UK has committed itself to allowing a legal backstop into its "departure agreement" with the EU to the effect that there will be no regulatory divergence on both sides of the land frontier.  That means no interruption of the movement of goods or people across the frontier.    There are only two ways to avoid that backstop.  Either there will be a full trade agreement which will cover the issue, or else there won't but the UK will present technological (i.e. non-physical) methods of