What happens about VAT for Northern Ireland will be a bit of a nightmare. As pointed out by many, including Nigel, the UK wants to be free to use a different system. For the sake of a name, let's call it Purchase Tax (PT), though it may differ significantly from the earlier incarnation of that.

So when I sell into the UK, I need to use PT, and when I sell into the EU, I need to use VAT.   And I also have to keep sufficient checks to ensure I do not ever accidentally apply the wrong tax to a product, o I need traceability right through from product, through warehousing and dispatch. Some components in the product may have come from an EU source, and the tax is reclaimable, and some from the UK, where whether there is any ability to reclaim is unknown.

That sounds a lot of bureaucracy added into each and every business, and VAT was always claimed to be heaviliy bureaucratic on its own.