James, I think for most it has always been difficult to make a living from instruction over he years, worst still these days instructor pay does not seem to have increased in line with inflation. To add to Okavango comments on today's, when I was a full time PPL instructor AFI then QFI in 1988, I was earning £12000 pa with medicals, renewals, night instructor rating NI and holidays paid for.
The overheads of instructing has increased, not helped by pointless instructor seminars, I gave up part time examining because of the increased overheads. Long term there will always be airline pilots both employed or retired who really do not need the pay, likewise those who need the flying hours. If you think about it £20 to £25/hr is not that much based on your costs and say the average number of flying hours you will do in a day (if you are lucky 3 to 3.5 hrs or zero if the weathers poor.
My advice is do not give up your day job, instruct part time and see if you can break even, or make a loss and set that off against other taxed income. I think if you treat instruction as a hobby, you will have fun and keep your sanity.