“Blockchain might solve Ireland border problems” – UK Minister of Finance

One of the major reasons the UK decided to severely injure itself (sorry, to "Brexit") is that they wanted to eliminate free movement/migration, primarily from Eastern Europe, to the UK. This is permitted today due to the EU's principal of freedom of movement between member states.

During exit talks, the EU, however, told them there's no world in which they get to pick and choose from what they consider the 'four freedoms' (movement of goods, movement of labor, movement of capital, movement of services). This is not something they thought would really happen - they overlooked the fact the EU27 had every incentive to nail the UK to the wall to remind the remaining member states they don't get to pick and choose either, whether they stay or go.

This wouldn't be a big deal in and of itself, the UK is an island right? Except... along the border between Northern Ireland (EU27) and Ireland-Ireland (UK). The place they had a decades long civil war ("the troubles"). Nobody along that border wants to see a new physical border or checkpoints constructed. However, the EU won't allow goods to move freely unless the UK remains in the customs union/single market/single immigration region. Leaving this was a core premise for Brexit too. They're now backed into an epic corner, where they can't figure out how to have a border for goods, but not really, and not for people, but also for people. Hence their appeal to magic.

This all came to a head recently when the UK PM retreated to Chequers with some top cabinet ministers to put together a last-ditch proposal to make a deal. Appearing to her hardliners as giving up too much, a few left her cabinet (Boris Johnson, for one). Proposed to the EU27, they told them, again, they don't get to pick and choose freedoms, and to stuff it. I think the core solution proposed was to get EU customs officers to charge UK duties on goods arriving at EU ports destined for the UK? I can't remember exactly, it didn't really make sense. Hence this stroke (of genius?) from Hammond, probably as a way to track duties paid as nobody trusts each other.

This is going to get worse, yet, once the gaze shifts to Gibraltar. The tiny little British Overseas Territory hanging off Spain. The same issue exists there with goods. And people. In fact Spain has been demanding the return of Gibraltar for decades, but once they were both in the EU, and people and goods could move freely anyways, it didn't matter. Prepare yourselves, it's next.

tl;dr: Brexit has been, and remains, a national-level popcorn thread for years.

Hope that helps.