And also leaving afterwards. Erasmus is not a migration thing, it's purely temporary.

Not to mention vice-versa - young Britons living abroad and seeing different cultures and ways of doing things.

I can tell you from my experience as an immigrant (in several countries, including Britain) that there is no better way to start spotting that so many things in one's homeland aren't thus because "that's just how these things are", than living somewhere else for a while and seeing many such things being different or done differently.

All countries have plenty of shit with massive negative effects or side effects - not just politics but actual cultural habits and even the way people work - that needs not be so and is done better elsewhere.

For example, Britain's idea that "work hard" is a good thing is almost the opposite of the view in countries were people work much more efficiently - and thus, amongst other things, have way better work-life balance - such as The Netherland, were the objective is "work smart" not hard and people working long hours are suspected of doing so because they don't know how to do their jobs properly or management is incompetent and doesn't know how to plan and resource correctly, almost the opposite of Britain were it's usually considered a good thing that shows commitment.

For the kind of peoplo who manipulate the muppets who vote for things like Brexit, people openning their eyes and seeing how much of the shit going on in their own country that they think is "normal" is no such thing, is a threat.

Bloody foreigners coming 'ere and stealing our education!