I don't know why instructors are not paid well in the UK but those of us who fly in the US and are charged for the instructor's time, even for the pre flight and the logbook signiture, understand that they don't work for nothing.
Looking at a school here, £139.00 student rate per hour and the instructor gets £12.00. They could almost earn that frying chips in McDonalds.
I would pay good money for a good instructor and so would most people.
TR