To answer question:
Somewhere between what a bus driver (c20k) and a train driver (c40k) earns.
That's what I have aimed for here and we have had one instructor with us for 12 years, another for about nine years.
Key is - we teach NPPL Microlight. So the capital investment in getting an instructor rating is low, think about 8k. Twelve years ago, we actually loaned our first instructor the cash for his course.
I am led to understand that the UK CAA have agreed, in principle, to raise microlight AUM to 600kgs. This will be a big change!
The only way you'll make a living - even at these sums - means being close to airfield, so little travelling expense. But many airfield are near nice places in the countryside, great for bringing up families!
So, it goes back to life is not just all about money - there is quality to be considered too.
Finally, just do some sums. Factor in Scottish weather and you'll be doing well clocking up 500 hrs in a year. Charge 200 an hour and that is 100k a year gross income for the flying school. So it can hardly offer to match "the other side of 100k" in salary.
Flying lessons are, simply, too cheap to pay a decent wage.