One of the 'boxes-in-boxes' was the arrangements for Northern Ireland. When both the UK and the Republic of Ireland were in the EU, there was no need for a cust

One of the 'boxes-in-boxes' was the arrangements for Northern Ireland. When both the UK and the Republic of Ireland were in the EU, there was no need for a customs border of any kind, and so one side of the NI community could pretend they lived in a united Ireland, while the other could pretend they lived in the UK, as no obvious border existed on either side. When the UK left, this became a problem, as a customs border would need to be erected somewhere. Thankfully the UK was forced into putting it in the only reasonable place, in the Irish Sea between NI and the rest of the UK, so limiting the damage to the Good Friday agreement. Effectively NI has been economically annexed by the Republic - formal unification is probably not too many years away. It would do you well to appreciate some political history before 'feeling confident' about things like Brexit.