3 years before you were born the UK had a big vote to decide whether it wanted to pay to be in a club of countries where anyone in the club could move to live in UK, and the UK had to follow all the clubs rules.
The people who wanted to leave the club thought we were capable of living the world without having to follow all the clubs rules, and saving the membership fee, but those who didn't tried to persuade them that the wider world was too scary to go alone, and that without the clubs rules we wouldn't be able to survive. This painting of it being too scary to leave was named Project Fear.
The main person who wanted to leave (Nigel) left the implementation of this to his friend (Boris). Boris, however, decided he prefer to have parties than to go and explore the big wide world. Now Nigel has found a new group of friends aren't scared of the big wide world and improve the UK place in it.
Some people miss being told what to do by the club the club and moan about not being part of it, but they won't move to be part of it because then they wouldn't get as much money from the government for sitting on their bottom.
The club is now so unpopular that lots of people try and leave it in small boats which is something that Boris didn't care about because of the parties, but Nigel will sort.