Bosses wanting to use prison labour rather than attract workers by improving wages and conditions:
Meat processors and others say they must have more day release workers as they cannot find enough staff
www.theguardian.com
I suppose this isn't really the same thing as the big scandals about unpaid prison labour in the US, but it seems like a form of undercutting all the same. Apart from anything else, I imagine that if you're on day release and your chance to be outside of prison depends on your continued employment, you are not going to be that likely to rock the boat with anything union-related or the like?