The bone of contention is people arriving in the UK and having access to unemployed benefits, housing subsidies and medical care all for free. This is sorely abused by people coming from places such as Poland and Romania which have not got a similar welfare state, and milking the freebies for all they are worth. With much of the benefits being sent home to family in theses Eastern European countries.
If a waiting period was introduced before access was given to these benefits, then the situation would level itself out as economic migrants would have no option but to find work rather than living on handouts.