Amending this post a little bit, I do understand the opposition by the people in Britain to the somewhat abstract concept of the EU, but not really any (most) elite opposition to it.
The legitimacy of any of these claims can be argued, though I think some have some value, but roughly speaking the sense in Britain is:
1) For various reasons it's a lot easier for people to migrate there within the EU than to other EU countries. One reason is simply that English has become the standard second language that everyone learns. Another is that the bureaucracy is a bit simpler there than elsewhere. Another is that London has a longer history of being a cosmopolitan immigrant-welcoming city. tl;dr: it's easier for most Europeans to move to London tham it is for them to move to Paris or Rome. They move here and take our jobs.
2) Britain obeys the rules more than other countries do. Brussels dictates something, Britain obeys, other countries don't.
3) All the immigrants are taking all of our benefit money, getting free housing etc.
4) Immigrants are horrible, they should all go away.
I guess I've ranked these in the order that I consider to be legitimate. 4) is obviously totally subjective. 3) is largely BS but not totally. I think there's a weird bit of truth to 2). 1) probably is mostly true, though the last sentence is problematic.