I don’t know if this is true in the UK but in the US McD’s locations are nearly all franchises owned by local business owners. So McDonald’s the corporation could reasonably be unaware that the 15 (say) locations in Dallas TX owned by Horatio Hornblower were all staffed by trafficked persons.
I’m not saying this would absolve McDonald’s of responsibility but it would address the “being unaware” part.
Sadly, it might end just fine for the boss. The employee would be better off going to the press first.