//i dont understand why brexit uncertainty would cause this - surely brexit is about our relationship with the EU and theirs with us, not about sending things between 2 points in the uk?//
Then you haven't been keeping up for the last four years, bednobs :-)
As Corby helpfully points out, once the so-called "Trade Deal" with the EU is in force, Northern Ireland will, de facto, remain part of the EU. It will remain in its Customs Union and people wishing to send goods there from the rest of the UK will have to comply with the same rules they would if they were sending those same goods to the EU. I don't know anybody in NI but if I did I would be somewhat miffed if I had to complete a customs declaration before sending them a box of chocolates. I've prattled on enough about this over the last couple of years but if the EU is concerned about contraband crossing the border from NI into its its precious Customs Union in the south then they should compel Ireland to erect border controls along the border. But instead they have persuaded the gutless UK government to do so in the Irish Sea. The idea of a border in Ireland is, apparently, unthinkable. Well the idea of one in the United Kingdom is not only unthinkable to me but it's an absolute disgrace. But it seems we're stuck with it.