The UK starts dumping non-EU-compliant goods into the EU customs zone through Ireland, undercutting EU producers. ~~The EU moves to suspend Ireland from the Customs Union.~~
Europe decides that the UK has fucked around enough and should find out.
France moves street signs in Calais, starving the British economy of Polish workers.
Spain closes itself to English tourists.
Germany threatens to create a financial transparency register, causing international oligarchs to panic.
The government relents and gives northern Ireland to Ireland.