Its all about Ireland this, they are the buffer, the invention of a middleman - perfect if you own said middleman and his means of moving goods back and to.
the UK exports more goods to Ireland than to the rest of Europe put together - so what is going to the rest of Europe goes through there. Its the vice versa that is in question - and a big part of why so much pressure was put on the Scots. Westminster calls the shots across the board, no Scotland doing deals on the backfoot.
So many moves ahead. Should the UK step out of Europe, who ends up hurt? Germany and France have to prop up their distant vagabond neighbours, the rebate disappears but the UK aint throwing in no more (Hopefully the fishing quota gets sunk along with some French and Spanish vessels) the economy bill and its associated late payment fees is the cherry on the cake - this move puts huge pressure on the Euro, and its newest leeches. Will the Euro membership costs be thrown at border protection and deporting the worst of the worst that have made the Uk their home (the current European arrest warrant debate/fight).
Edit: Ironically, this set of moves has the UK closer to Russia than it has ever been before.