They put you in an umbrella company and take the employers part and the employee part of the social insurance off your salary. On top of that you have to pay taxes in your own country if not UK resident.
Plus the cost of living and full fare tickets to commute as they offer not any ID ticket other than jump on own aircraft there is nothing left at the end of the month. Thats why p2f, you have to bring own money to survive even as a captain.