I knew another Polish guy that was there, for example if he earnt 100 then the crappy accomodation (an old touring caravan) cost 130.
That's sick indeed. Those people must feel pretty hopeless and helpless.
I wonder what I could do here? The thing is, what if people dont want to be 'rescued', when that woulld mean more homelessness
I think you could try to talk to them, inform them about their rights, I don't know, maybe give them a phone number to some normal job agency that wouldn't cheat them?
If they don't know English (or even if they do know some) it would probably be a good idea to give them those phone numbers:
Barka is a Polish NGO that has a branch in the UK helping Central and Eastern European migrants:
A helpline of British police for Poles (one can speak in Polish, apparently):
Maybe there's also some local helpline like this one:
Other useful phone numbers and addresses:
Is there any Polish community where you live? Some Polish church or sth?