EU opponents demand national independence for their country, so that it can determine its own laws (e.g. on immigration, currency, defense, fishing) and conclude free trade agreements with all countries throughout the world.
On June 23, 2016, the citizens of the United Kingdom decided decided in an ad hoc referendum, that the UK shall leave the EU (Brexit; 17.4 million or 51.9% of the voters). Since then, the country has been negotiating its withdrawal from the EU, wich, by decision of the UK Parliament, willl happen on March 29, 2019. The withdrawal agreement negotiated by Prime Minister Theresa May was rejected in the House of Commons on January 15, 2019 by a large majority (432 to 202), leaving much uncertainty as to whether and when Brexit will be implemented.
The biggest obstacle to a true, constructive, mutually beneficial Brexit has been, on one hand, the EU's - in particular its Commission's - "arrogant intransigence", which denies any UK "cherry picking", to discourage all other member countries from following suit; on the other hand, the deep division in the British population and the fierce resistance of Brexit opponents, who dominate in the British parliament and government and who want to tie the country as close as possible to the EU.
The Brexit opponents are prophesizing total chaos in the event of a disorderly WTO Brexit, the Brexiteers see in the PM's negotiated deal the betrayal of democracy and the permanent submission of the United Kingdom as a "Vassal State" under the dictate of the EU.
EU's opponents feel themselves affirmed that the EU has neither the will to fundamentally transform itself into a more democratic, subsidiary, decentralized Europe of the citizens, at variable-geometry (EU *), in which each Country can decide in an ad hoc referendum whether it wants to participate in a politically integrated core EU, or in the common market (without euro) or in a large free trade area.
Are more Brexits looming?
So far, the 27 remaining EU governments have supported Brussels' intransigence; and EU-critical parties in various countries (such as F, NL, I, H) stopped threatening to withdraw from the EU or the Eurozone. Some of them now hope to reform the Union "from within", with a more critcal Parliament after the European elections in May 2019. Should these hopes for internal transformation however not be met, other withdrawals with national ad hoc referenda cannot be excluded.