Mmm.. easy to say, less easy to do. If the school you work for refuses to pay more, and your nearest other school is 70 miles away, are you going to move house and start all over again at another school? Then have a nightmare commute with the associated petrol costs? I doubt it.
"any good school, who cares about it's instructors" - Mmmm.. schools know that instructors are always around and, on the whole, people only do it to build hours before moving on, so why pay us more? If they paid us all £70, then would have to put their prices up, and hence the customers wouldn't come.
It's all down to economics. Fuel, regulation/admin, airfield etc costs have all risen over the last few years, and schools are forced to pay those costs, but they can get away with not paying instructors more because they know there will always be people looking for instructor jobs.
Maybe in ten years schools will be forced to pay us more, but by that time, we might not have a PPL flight training industry in the UK.
I'm all for solidarity and getting a decent wage, but as I said, it's depressing.