Some will gather outside the Houses of Parliament to celebrate at 11 p.m. (6 p.m. ET) what they see as a new dawn for a buccaneering, independent U.K., free of undemocratic control by Brussels. In reality, almost nothing will change overnight.
Former senior civil servants and academics have told NBC News that this second phase will be the toughest part of Brexit yet, with the U.K. forced to make concessions and compromises along the way on trade and security.
Prime Minister Boris Johnson, whose slogan in December’s election was “Get Brexit Done,” has promised voters he will sign a comprehensive deal by a Dec. 31 deadline, without the need to extend.
The U.K. will still be subject to all E.U. laws and regulations until at least the end of 2020 and this transition period could be extended by up to two years. And the U.K. faces a more daunting task: securing an agreement on its future relationship with the E.U., including the finer details of trade, security and data sharing.
“I am yet to meet anybody who thinks a comprehensive trade agreement would be negotiated in 11 months,” Brexit expert Katy Hayward of Queen’s University Belfast, said.
The European Commission — the trading bloc’s executive branch — must get a mandate from the 27 remaining members before negotiations can begin.
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So getting Brexit “done” will not be simple.
“After Jan. 31, the government will take the line that Brexit is done and what remains to be done is a sort of technical exercise,” said Sir Simon Fraser, a former permanent secretary, the most senior civil servant, at the Foreign Office and a former chief of staff to the European trade commissioner.
“There’s no way that we are going to achieve by the end of the year a-once-and-for-all comprehensive set of agreements with the E.U. about our future relationship.”
Fraser said that the British public, fatigued by Brexit, “will be quite keen to go along with that.” But if progress toward a trade deal stalls, that relief could turn to worry if significant progress isn’t made, he added.
Joe Owen, an analyst at the Institute for Government, a London think tank, and author of a report about Brexit, said Jan. 31 was important symbolically. The U.K. will cease to have a seat in the European Parliament, for example.
“But the follow-on questions about what Brexit means are largely still unanswered,” he said.
“We don’t know what our new immigration system will look like; we don’t know what we’ll do with an independent trade policy; we don’t know what we’ll do with agriculture and fisheries; we don’t know what we’ll do with devolution in the U.K. and the split of power between Westminster and Scotland, Wales and Northern Ireland.”
The British government has suggested that an agreement can be sealed without submitting to the so-called level playing field, whereby E.U. member states and trading partners share the same rules on environment, labor laws, tax and state aid, designed to ensure fair competition.
If you don’t agree to that, “you cannot have the highest quality access to the world's largest single market,” Ursula von der Leyen, the new president of the European Commission, said in London earlier this month.
In other words: if the U.K. wants to set different rules, for example to allow hormone-fed cattle imports from the U.S. — which are banned in the E.U. — then there will be consequences for a free trade deal.
British Finance Minister Sajid Javid boldly told the Financial Times on Jan. 17: “There will not be alignment, we will not be a ruletaker, we will not be in the single market and we will not be in the customs union — and we will do this by the end of the year.”
After some negative reaction from British business leaders, he dialed down the rhetoric at the World Economic Forum in Davos, Switzerland, by saying the U.K. wouldn’t diverge from European rules “for the sake of it.”
This matters because the 27 remaining E.U. countries each have the power to block new treaties and don’t want to see the U.K., a longtime neighbor and now a competitor, prosper while breaking the rules they all have to follow.
A comprehensive, 1,600-page treaty between the E.U. and Canada eliminated 98 percent of trade tariffs between both sides but took a full seven years to finish. The treaty, worth billions of dollars, was ultimately held up by the Parliament of Wallonia, a French-speaking Belgian region that has the power to stop Belgium ratifying E.U. treaties.
“The reality of international relations is that if you are the U.K. and you’re next to the E.U., you’re going to be in a permanent set of ongoing negotiations — as things change in Europe, you have to adjust to them and you have to decide how you react to new regulations,” Fraser said.
“So, yes, we will be in a permanent ongoing negotiation with them anyway.”
Patrick Smith is a London-based editor and reporter for NBC News Digital.