Instructor Salary?
You should expect to earn from £zero to £15 per hour. You would be being optimistic to earn any more than £10,000 - £15,000 per year where you are are paid. You must hold a CPL to be paid as an Instructor. If you add an Instructor Rating to a PPL, then you cannot be paid.
Instructing is unlikely to pay off your loans. Your later salary as a CPL will need to do that. Not really fair, perhaps, but reality