You think that's bad, should see how much the airlines charge me for extra leg room, or seats together.
To be serious, the charges reflect the way GP's are paid. (They are not contracted to provide sick notes). The system is a little a archaic, and soon the system will focus on "patient quality" issues, rather than "ticking the correct box."
Some GP's will issue sick notes for free, at the moment you have some degree of choice over who your GP is, vote with your feet, and tell your GP why.