Coronation I think you have been reading too many rightwing blogs.
This country is not so wonderful that anyone that studies here wants to stay here. Foreign students come from all over the workd, Canada, the USA, every European country, Australia, new Zealand, japan, China as well as those nasty places like Africa and Asia, and all most of them want to do at the end of their studies, even if they come from nasty palces in Africa and Asia, is go back home to their families and to have successful careers in the comfortable economies they come from where, frequently their health service is a darn sight better than ours, as are their education systems, and housing is cheaper.
As for the families, if someone sent you or your husband to another country to work or study for three years, how would you feel about being separated for that long, especially if you had small children?
The majority of students, especially the young ones are not married so the question dooesn't arise.
The vast majority of immogrants come here quite legally to fill jobs in the NHS and Care system. The vast majority of students (and their families) go home at the end of their studies. Hopefully with happy memories that make them think kindly of us when they reach posts of power in companies and government in their country.
haven't you notice when heads of government and senior international people visit here, how many spent part of their education in the UK? - and how few of ours have.
Illegal immigrants ar a tiny fraction of the total entirely legal welcomed immigrants. Stop all these al;armist talk unless you have clear datat to back your assertions.