My advice, all instructors stand together and state they want £50/hr. No haggling. If enough instructors do this at once then something might happen. It needs to be higher as the cost of getting the licence gets higher and higher each year.
My instructor back in 1994 was getting paid £45/hr!!!!!
Saying that, any good school, who cares about it's instructors, will pay them £50/hr. No reason not to. If they don't then maybe look for someone who does pay well. You want to work for less than what you think you're worth, then go ahead but moaning about low rates won't get you anywhere i'm afraid. Take action!!!