The "free movement of labour", was a cynical ploy by the rulers of the EU to provide cheap low maintenance labour to the richer members.
The effects of this migration on the infrastructure of these Eastern European countries is bound to be dire.....who runs their Public Services, who builds their homes?......Only the never to be paid back bribes from EU coffers to soften the disintegration.

Our own people are too expensive to retrain, so we have hit on the ploy of squeezing the benefit system which was used as a tool to keep people quiet when their employment was recycled.

The benefit culture is a disgrace, it demeans humanity, but there must be something to replace it and we must learn to live within our means...and to attain status without obscene wealth differentials.

In answer to your question...."slave labour", victims of exploitation.