It's tricky, this subject.
A close friend of mine used to work in a factory (you would know the name of the product) in the late 90s and early noughties on a rolling annual temp contract. It was in an area that was notorious for poor employment opportunities, and the work force was entirely British. Then, over a period of six months, management stopped renewing contracts for the British workers and the workforce was entirely replaced by Polish EU migrants. My friend lost his job after working there for nine years, as did everyone else. The British workers were just thrown on the dole.
My friend took them to a tribunal over it and, unfortunately, lost. His belief was not that the factory thought they could pay the migrant workers less per se, but more that migrant workers were not aware of their rights in terms of overtime, health and safety, bank holidays, regulatory breaks etc so they were easier to deal with and there was more leeway for the factory to operate in those "grey areas". Whereas a British worker would have raised a health and safety concern over being expected to load very heavy weights onto pallets for an extensive period of time, the migrant workers just did it and never said anything, even when they threw their backs out or suffered injuries.
I've come across the same story elsewhere, particularly in factory environments. It's not that these employers can pay less in hourly wages, but more that they can get round other types of regulations with migrant workers.