On another topic: it's a good wheeze of Gove's not to have the checks on lorries coming into the UK for six moths or so after Jan 2021, is it not? Even though the EU will undoubtedly retain its checks to main the market integrity.

Let's think of lorries and flows. Kirchhoff's Laws and all that. The net flow of lorries across the Channel must be zero. If not, we either run out of lorries entirely, or they all end up in the UK.   By the same law, the rate of lorries leaving a queue must match or exceed the number joining it (over a suitable time interval), otherwise the queue becomes arbitrarily long.

This means having no checks on goods coming into the UK ultimately doesn't help: the dominant term will be the greater of the EU and UK handling time.    As a rough and ready approximation, there are no lorries available to bring goods into the UK, because they are all stuck trying to get out of the UK.