Much is made of the easy Norway-Sweden border, but both states are members of the single market but also the Schengen free travel area. Even so, there are controls and checks, and there would be many more if either country had the British phobia about migration.

The reason why the Irish border issue hasn't been sorted out more than a year after the Brexit referendum is that it cannot logically be the same as it is now - frictionless and seamless. When the UK leaves the EU customs union, with or without transition arrangements, some mechanism will be necessary to certify origins, to ensure that goods imported into the UK cannot travel into the European Union, ie Ireland, without some notification of their origin and whether they conform to EU rules and have paid EU duties, and vice versa. Otherwise the EU's common tariff barrier and the rest of the world cannot work. Modern technology and licences granted to trusted companies can help assist this, but the fact remains that some fresh bureaucracy, even if mostly digital in form, will be required, and human beings will be needed to police it.