Buy a type rating save the hassle
I earnt £10 per hour in one school and £20 per hour in another, then got retained but with no hourly rate on top and have seen instructors earning £35-45k doing CPL/ME IR whilst other instructors in the same school earnt £12ph if the weather was good that day with no expenses or retainer.
At the end of the day most of schools making you work as self employed know that they are breaking the law from an IR 35 perspective but to be honest if you were to point that out chance were you would never work as an Fi again anytime soon in the region so I would just negotiate whatever you are able and either have another job or set a time scale whereby you will leave unless you happen to have a private income or live at home with friends or family and do not need to contribute.
Everyone knows that it is not a living wage which is why guys go out and buy type ratings as its cheaper than living on an FIs salary for 18 months plus when you tot it all up.