The EU opponents in the UK - led by the United Kingdom Independence Party (UKIP) under Nigel Farage (member of the European Parliament 1999-2020) - has been asking since the early 1990s that the UK leave the EU to achieve national independence, so that it could determine its own laws (e.g. in matters of immigration, fishing, tariffs) and conclude free trade agreements with all countries of the world.
In 2013, in order to secure his government, Prime Minister David Cameron promised to negociate with the Brussels to get special rights for the UK, and then to hold a politically binding referendum on the country's withdrawal from the EU. In February 2016, Cameron praised the compromise he had achieved with the EU; and recommended to remain in the EU.
However, following a fierce campaign, in which Nigel Farage had played a key role with UKIP, the EU opponents narrowly won the referendum on 23 June 2016: 17.4 million or 51.9% of the voters supported Brexit, i.e. Britain's exit from the EU, with a participation of 72.1% of all entitled voters.
This historic decision signalled the start of seemingly endless, chaotic Brexit negotiations between the UK and the EU. At long last, on 29.3.2019, the House of Commons rejected (for the third time) the "Brexit Deal" negotiated by Cameron's EU-friendly successor, Theresa May, who therefore was forced to postpone this (first) departure date.
On June 24, 2019 - three years after the referendum - Boris Johnson, became the first Prime Minister supporting Brexit. In October 2019, the Lower House supported, in principle, Johnson's new, partly improved agreement with the EU ; but forced him to postpone the (second) withdrawal date of 31.10.2019 as well.
On the basis of documents from the UK government, think tanks, banks and big business, Brexit opponents have been cautioning, in the case of a hard Brexit (according to WTO, without a deal) against total chaos in the UK.
Food and drug shortages, traffic break-downs, the loss of entire sectors with dramatic economic consequences, heavy unemployment and a long recession were prophesied. At the same time, these remainers have been warning of exaggerated expectations from the hoped for new trade agreements with third countries (Commonwealth, USA, Japan, ..).
Conversely, brexiteers calling for a hard, "credible" Brexit, in particular Nigel Farage and his newly founded Brexit party (winner of the 2019 European elections in the UK with 30.1% of all votes), considered the new deal negotiated by Johnson to be a Brexit in name only, a momentous betrayal of 17.4 million voters, disastrous for the British democracy altogether, as well as the UK's permanent subjugation as the EU's "vassal state".
Brexit opponents, who have dominated parliament, Theresa May's government and the British civil service, have succeeded in virtually blocking the negociations for more than 3 years by doing everything they could, incl. contacting Brussels, to undo or delay Brexit and to keep the country as close as possible to the EU - even though before and after June 2016 both government and the Labour and Tory parties had steadfastly been claiming the citizens' vote to be binding (including in their 2017 election manifestos).
On the opposite side, the EU has been acting much more successfully and coherently: In order to avoid, by all means, the exit of other countries, resulting, possibly, in the decay of the Union, the EU's chief negociator Michel Barnier and his team meticulously made sure that the UK would not get an attractive agreement and definitely no "cherry picking"! Thanks to this impressive unity of the EU and the remaining 27 member states, this strategy worked perfectly.
The turnaround came with the general elections on 12.12.19. They gave the Tories a landslide victory with the slogan "Get Brexit done" - also thanks to the support of Nigel Farage and his Brexit party.
On 31.1.2020 the UK formally left the EU; it wants to sign a free trade agreement with the Union by the end of 2020. Whether this can be achieved remains to be seen.