I think this is the whole problem with NHS, as Lincoln explained. If you, as a patient do not have a choice of dentists and can't go private and your NHS dentist won't prescribe you any oral sedation, but you have to get the treatment somehow, then I can see going to GP as the only hope. It's a circle. I don't think GPs prescribing any meds for dental treatment is right.
In the country I live in we do not have any possibility to go private, the whole country works on a NHS-similar system. This leads to phobics and dentists using GA as the general solution because no dentist anywhere has the time to even talk to the people about their fears.. leads to plenty of people who got everything done in one go on GA and then stop going for years again. Even doing tell-show-do for every treatment would take too much time as well so in the best case the dentists manage to got at least tell-do while treating. Coming back to the original topic, if the system is not set right to offer support for phobics, then counter productive things start to happen.