The EU is deeply unpopular in the UK and other European nations. The British people never voted to join the EU, and if there were a referendum, I have little doubt that the result would be Yes for the UK to withdraw (secede) from the EU. The EU has brought about a resurgence in nationalism, and rightly so.
That no referendum has taken place is profoundly unjust. Membership of the EU should be decided upon by the British people, not by politicians, nor by anyone outside the UK.
But why stop at the secession of the UK from the EU? Consider Scotland. Who should decide whether Scotland should secede from the UK? To be consistent, it should be the people of Scotland, not politicians, and not the people of the UK. If the Scottish people want to secede from the Union, it is unethical for anyone to prevent this. Membership of the UK should be decided upon by the Scottish people.
The same principle applies at an even more local level. If the people of the county of Cornwall, or Yorkshire, or Buckinghamshire wish to secede from the UK, or the people of the city of Oxford, or Liverpool, or Bristol wish to secede from the UK, it would be unethical for anyone to prevent their secession.
Perhaps you can see where this is going. If a small village community, or a family, or a single individual wish to secede from whichever state they are a subject of, who should decide? Well, the secessionist(s) themselves, of course, and not the entity they wish to secede from.
If one opposes the EU and believes in the right of secession at all, they must recognize the right of individual secession. That is, they must recognize the right of any individual to stop paying taxes and to stop following the rules and regulations forced upon him by the state, whenever he wishes.
Nationalism, while preferable to internationalism, is a blatantly inconsistent doctrine. The only consistent positions are a one-world government (which is justifiably, almost-universally unpopular) or individual secessionism, also known as libertarian anarchy.