30 pounds for mitigating circumstances?

Is it normal for a GP's clinic to charge £30 for a sick letter? I recently suffered an injury and needed a letter stating my mitigating circumstances. However, I was confused when the recipient informed me that I needed to pay for the letter, despite already having letters from the hospital. I felt like I was being scammed.

Also I am a bit confused should I apply for course works extension or mitigating circumstances letter of If I suffered from an injury and need more time to submit my assignment?