Cannot answer your second Q, MBM, but with reference to the first, I think it must be the old swings and roundabouts scenario. A woman I worked with was on a certain medication, which apparently cost the NHS �30 for 1 months treatment, but she only paid the �4+ prescription fee.
My (previous) GP would always tell me if it was cheaper for me to buy the suggested medicine etc over the counter, instead of through a prescription from him. Of course that was for limited types of treatment, not the ones only available through a prescription because of their content.