well like I said, im not a brit, let alone a european so my knowledge can be lacking but I’ll still try to give my perspective until someone else comes alonng and gives a better explanation. The problem from my POV is that once rules are set, EU memberstates are practically stuck following them unless theres a change of government in the memberstate where they refuse to follow or at least certain EU demands etc like in Italy for immigration. And for the UK, immigration is a concerning thing especially after 2015 where over 1 million entered Europe with most going to Germany and Sweden, while Britain took in less than 50,000. Its more like out of fear due to crime committed which alot tend to be of migrant background. The result of brexit alone was actually mainly influenced by immigration rather than shit like economics but still plays a big part.