Depends on their level. If they're in demand (top scientist/engineer/doctor etc) then get them in no matter what. Short-term? Doesn't really matter. Looking to come over, stay for a good few years, then should be making a contribution to society, so, yeah, then.
There are immigrant tests now. Pretty sure they must be taken in English, Welsh or Gaelic (may be wrong there but I don't think I am). I'm sure you can see them somewhere online. The questions can be pretty ridiculous as regards what it means to be a British citizen. I remember a colleague who was sitting the exam asking me how many countries were in the Commonwealth. I guessed about 20. Without looking, what's your guess. I'll post the answer later.
Obviously EU countries can come in no matter what but then we can also go to theirs without restriction, so fair enough. I'm also in the Middle East and do short-term work on places in Africa and the former Soviet Union. No language restrictions there (although a long-term work permit has pretty much the same restrictions as in Western countries).