The Problem is Smuggling
Brexit is all about having different import duties and/or regulations on either side of the UK/EU border.
As soon as there is divergence, there will need to be bureaucracy, much of which – and maybe most of which – can be automated. Legitimate, organised, compliant traders might face little or no monitoring, even if they find ‘rules of origin’ to be a right pain.
But there will be an incentive to evade higher tariffs and/or higher regulation by smuggling food, livestock and other goods across the border. The incentive will be in proportion to the difference between the UK and EU regimes. Low import tariff UK food, for instance, might be smuggled into the Irish Republic over any of the >200 current crossing points. But smuggling needs to be identified at the border, or else there will be little deterrent and no sound evidence trail which can be used by prosecutors .
Some traffic and some people will therefore need to be stopped and searched. This is why a so-called ‘hard border’ will be needed – in Ireland as much as in Dover and Calais.