The main reason depended on whom you asked.
As per what the majority of those who SAID they voted Leave in 2016, said in 2016, it was immigration by nationals of other EU countries that was the most given reason.
This is understandable - in the ten years after 2004 you had nearly 2 million Central Europeans moving to the UK. Central Europeans countries joined in 2004 and the EU gave countries like the UK, France, Germany, netherlands the ability to hold off on immigration for 4 years and also to evict EU immigrants who didn’t have a job in 3 weeks. Germany and the Netherlands implemented these rules. The UK didn’t. So people went to the UK to work.
The people who lost the jobs were largely the white-van drivers. There became the stereotype of the Polish plumber - a guy who could give you quality work at a low price any time of the day. Before 2004, when I lived in Sussex, you were terrified of hiring white van blokes - they’d overcharge and never complete the work
Wow.
Well it looks like a hard BRexit is coming now.
At least that’s the way it appears.