Do we know that these workers have low pay and poor working conditions?
It's very emotive to hear that a worker in x profession in x country only gets paid £100 a month, but what's actually important is what that £100 buys in their native country.
If they can feed ,clothe and shelter their families for a month for that sum then, in real terms it's actually the equivalent of a UK wage of much higher nominal value.
I'm not saying exploitation doesn't happen, it obviously does, just that detecting it isn't as straightforwards as just looking at a weekly or monthly wage directly converted into pounds or dollars.