It is the title of the Conservative manifesto, plastered on mugs, T-shirts and the Tory battle bus, while Boris Johnson doesn’t miss the chance to say: “Get Brexit Done”. Like an earlier slogan on a bus, Get Brexit Done is deeply misleading: the UK’s departure from the European Union is only the start of a new phase in the Brexit odyssey.

The day after

The day after Brexit the UK will embark on arguably the biggest negotiation of the post-war era: to reconstruct 46 years of trade, security and foreign policy ties with the EU. Philip Rycroft, the former permanent secretary of the Department for Exiting the EU, told the Guardian: “Obviously it’s going to be a huge negotiation, probably four or five times bigger than the withdrawal agreement negotiation and will absorb a huge amount of government effort.”

Trade is a top priority for both sides. Rycroft, who oversaw post-Brexit planning at DexEU ahead of the original 29 March deadline, said the government had already done a huge amount of work. Some on the EU side wonder whether it will be enough. Lotta Nymann-Lindegren, a former diplomat, who followed Brexit for the Finnish government, said: “The discrepancy between the two sides will be bigger in phase two because the United Kingdom has not negotiated any trade agreements in the past 40 years or so.” That “experience gap” will be “a challenge for the negotiations that will influence how fast we can go”, she added.

Deadlines

On Brexit day, the countdown clocks will reset to a new deadline. If the UK leaves on 31 January, only 11 months remain to hammer out the basics of the future relationship. Michel Barnier, the EU’s chief negotiator, told the Guardian that it would be possible to negotiate a basic free-trade deal in that time.

In private, Brussels is much more sceptical. “Not in my wildest dreams would I imagine that a possibility,” one senior EU diplomat said of the 11-month timetable, citing the difficulties of agreeing a zero-tariff, zero-quota trade deal if the UK seeks to diverge from EU standards on workers’ rights and environmental protections.

To woo Nigel Farage and Brexit party voters, the prime minister insists that he can negotiate the deal in 11 months, with no extension of the transition period. Labour wants a back-up plan and is pressing to avoid “the trap door to no deal” on 31 December 2020. Under the transition, the UK remains part of the EU single market and customs union, without decision-making power or representation. The government has until 1 July 2020 to agree with the EU a one-off extension of the transition period, until the end of 2021 or 2022.

If the next government seeks to extend the transition, it could soon run into trouble. Aside from a potential political backlash against “vassalage” and the inevitable extra payments to the EU budget that come with a longer transition, a decision on extension risks becoming hostage to a deal on fisheries.

The two sides want to agree future fishing quotas by 1 July 2020. The French president, Emmanuel Macron, once described fisheries as “a lever” in the future negotiations – a sign of how seriously EU governments treat this small, but politically-sensitive industry. EU diplomats do not exclude that a fisheries deal may be a quid pro quo for extending the transition. “Given how France acts now, it’s very likely that they will be a difficult partner,” another senior diplomat told the Guardian.

It’s much more than trade

Both sides want to prioritise security, such as a replacement for the European arrest warrant and access to crime-fighting databases that are used every day by British police. While negotiators share the same goal, the terrain is strewn with political minefields, such as Brussels’ insistence that database access is linked to EU rules on data protection and the oversight of the European court of justice, or Germany’s constitutional ban on extraditing its nationals to non-EU countries.

Beyond trade and security, there is … everything else. The political declaration agreed between Boris Johnson and the EU reveals what lies beneath the tip of the iceberg. The two sides want agreements or cooperation on aviation, carbon pricing, anti-money laundering, illegal migration, data protection, sanctions on rogue states, and much more.

The EU

The next Brexit chapter could be more testing for the EU. During phase one, the 27 had a shared interest in seeing the UK pay the Brexit bill and protect the rights of their nationals in the UK. Under phase two, their goals diverge somewhat: “The interests of the EU side are more diverse, whether [they focus on] industrial produce, whether it is the labour force they provide, whether it is fisheries,” suggests Nymann-Lindegren. Other Brussels sources are more optimistic about maintaining unity, suggesting that member states will be united against any attempt by the European commission’s trade department to run the British talks in secret.

What about schools and hospitals?

While British officials are racking up Eurostar miles, the main parties hope to return to the traditional domestic agenda on public services. But political time and capital will be spent on creating a new immigration system, laws on farming and fishing, competition and industry. Before the referendum, British sources predicted Brexit could dominate the annual Queen’s speech for several years after the vote. Under the withdrawal act bill, 46 years of EU law will be copy-pasted on to the UK statute book. “It would be very odd if the conclusion of all of that is we are not going to do anything about it,” Rycroft said. “It rather obviates the point of coming out.”

Just as the laws are repatriated, so are the controversies, especially over issues such as farm subsidies, GM crops or state aid. An extra layer of complexity will be tussles between Westminster, Edinburgh, Cardiff and Belfast (if the Stormont assembly sits) over who gets to repatriate powers from the EU.

Looking for Global Britain

So far, the government has managed to negotiate 18 “continuity” deals covering 48 countries and 8% of UK trade, according to the BBC. But these deals simply rollover existing agreements the UK enjoys as an EU member. For Brexiters, the prize is new trade deals, although the government’s own analysis shows that gains will be marginal at best. Looking at these “speculative” gains in the middle distance, the UK’s former ambassador to the EU, Sir Ivan Rogers, has said: “There is just an immense volume of technical work, even to aim to stand still, not roll backwards, in the next few years.”

Hard choices

The saga of withdrawal has obscured the debate on what Brexit means. EU officials worry that there is no British consensus on the post-Brexit future: whether it is a distant Canada-style free trade deal favoured by Boris Johnson, or the closer ties sought by Labour. Without consensus at Westminster, negotiations in Brussels could soon get stuck again. “The UK needs to have a majority for a vision of Brexit,” the senior diplomat said. “Otherwise we face the same problem. They need to know what their aim is.”

Donald Trump’s verbal grenade lobbed into the British campaign was a reminder that the UK has a choice: to follow European standards or embrace US regulatory norms. It is the chlorinated chicken conundrum: if the UK chooses to allow imports of US chlorine-washed chicken (and other produce) it will face much tighter controls on the food it can export to the EU, as well as price pressure on British farmers. There is no middle way between regulatory superpowers.

Those unanswered questions are why those who were involved in withdrawal hope that Brexit will be at the centre of the campaign. “I don’t think the public is ready for [phase two],” Rycroft said. He hopes that politicians “will level up with the public and give them a clear intention of what is coming down the track, because this story is by no means at an end”.