A reminder that Büsingen, part of Germany, is legally by way of treaty, part of the Swiss customs territory and Campeone d’Italia, part of Italy, is de facto part of the Swiss customs territory.
That’s no indication of what the UK will or will not do, but the “no country” would have part of its territory be part of the customs territory of a “foreign power” is wrong.
I neglected to introduce my tweet with “still necessary to repeat this” as that seems customary now.
