Theresa May had three goals. They were (1) avoiding a hard border on the island of Ireland, (2) no controls on goods flowing from Great Britain to Northern Ireland and (3) the UK's departure from the EU single market and customs union. The border between Ireland and Northern Ireland became hereby a Brexit trilemma. Where would the inevitable border checks on goods between the two sovereign jurisdictions take place? The law of trilemmas is that one can only attain two of three goals. May for a long time did not accept that she had to limit herself to two goals because she wanted to design a new model of frictionless trade with the EU despite the UK's departure from the single market and customs union. This is why the Joint Report of December 2017 put forward as a first option the UK's intention to solve the conundrum of the border on the island of Ireland in the future relationship. If the future could not crack the problem of the border, the report stated that the UK would “propose specific solutions”. The prime minister favoured a combination of those two avenues to avoid a hard border. The EU, however, refused to commit to these two options in the Joint Report in light of the UK's decision to leave the single market and customs union and the threat it posed for North–South cooperation on the island of Ireland. As a result, the only truly joint EU–UK commitment was Northern Ireland's alignment to EU rules, if all else failed. The formulation of May's preferences was vaguer, which many UK observers missed when the text became public on 8 December. They were things the UK “intended” to do or “undertook” to propose. The EU told May's team to hurry up with formulating its proposals.

British newspaper analysis added to the confusion by suggesting the report meant UK-wide alignment to EU rules. Given how talks unfolded in Brussels, there could be no misunderstanding that the use of the “Belfast veto” against alignment to EU rules, as put together in London, was an internal UK matter and that the agreed alignment concerned only Northern Ireland. It was not difficult for UK spin-doctors to convince reporters of the opposite. The text stated “the United Kingdom will maintain full alignment with those rules of the Internal Market and Customs Union which, now or in the future, support North–South cooperation, the all-island economy and the protection of the 1998 [Good Friday] Agreement”. The formulation reflected international law whereby the UK is the sovereign state that commits to obligations on behalf of its devolved entities.