Since when was £12 the prescription charge?  It's £9.  And the doctor doesn't pay it, YOU do at the chemist.  Haven't you ever had a prescription?

doctors don't pay anything to write you a prescription.

£ indicates the UK. I don't know how the UK healthcare system works, but doctors should NEVER pay a fee to prescribe mediation.

Prescriptions don't get fulfilled.  They get filled.

Other than that, I don't know why a Doctor would pay a fee to write a prescription.