As GtE says:
If you want to be an instructor, look at the microlight route.
To get a microlight licence, £5k.
Buy a second hand factory-built C42 - I know of some with around 700hrs on them for around 35k.
Fly to build up 100 hours P1. Cost you about 7k in fuel, insurance and depreciation.
Do a microlight instructor course. £4k - might get a discount using your own aircraft.
Then work as an AFI with a local school. If you also have your own aircraft, then it makes life a lot easier.
One year on, if you have 100 hours as an AFI and, ideally, taken a student or two from start to finish, get upgraded to an FI.
Now you can even set up your own school. Or just keep working part time at a local school with your own aircraft.
Total: £51k minus some income and an aircraft still worth 30k!
That'll be why Flight Training News says there are more microlight schools than GA ones.