Well, the law is the law, and if companies are paying less than minimum wage, then the migrant (not immigrant) workers can do something about it for up to 7-years following the date of the offence.
That's the first point.
Secondly, anyone who KNOWS that companies are employing migrant workers at less than statutory rates, they have a public duty to report those companies to the Dept. of Trade & Industry.
Thirdly, I would encourage ALL migrant workers to join a trade-union, which the employers may not like, but about which they can do nothing.
So your question and related comments are not really valid, because it has nothing to do with a lack of LAW that the rules are perhaps being broken, but a question of CRIMINALITY on the part of rogue employers and especially on the part of bad employment agencies.
Don't just moan about it....gather evidence and get these employers prosecuted!
The Eastern Europeans are great people by and large, and they deserve to be treated fairly.