Whilst I have some sympathy for your situation, you have to remember that there are currently more FIs than are needed to meet the demand from customers wishing to learn to fly. This has been exacerbated by some FTOs continuing to turn out FI(R)s who have litle chance of securing work. Hence 'market forces' are currently conspiring to make the demand for new instructors fairly limited.
Assuming that a fleet of 5 aeroplanes flew 600 hours per annum, that would make a total of 3000 hours. To pay a Flight Instructor a salary of £30 000 per annum would immediately raise the cost by £10 per hour - a minimum of £450 per PPL applicant. I doubt whether many would be happy to accept that....
So, regrettably, until there is a significant shift in the airline pilot labour market to stimulate demand for more new FI(R)s to replace those moving on to the airlines, it is unlikely that the situation is going to change and the 'benefit in kind' current low-paid FIs receive from the flying hours and experience gained from instructing is likely to be there only tangible reward.
The alternative? FTOs and RFs will be far more selective in choosing their FIs, given the number who are currently chasing a finite customer base. Those unsuccessful would then have to pay for their 'hours building'...............
The current situation is far from ideal; realistically it won't get any better in the short term and you should view your pay, low though you might consider it, and your 'free' flying hours as an apprenticeship. Sorry, but that's the brutal truth.