UK dentists have a limited prescription list, it only contains drugs that would be associated with dentistry. They can prescribe Diazepam and a couple of other benzo's, but probably only 2 or three pills at a time and you have to be requiring them for dental treatment about to be carried out. In most cases, I would expect they would refer you to your Dr for initial prescription as they will know your full medical history. Only if you were well known to the dentist and your Dr had prescribed them before would the dentist issue a prescription, I would expect even in this case they would still check with your GP. Antidepressants are not required for dentistry so I doubt they are on the permitted list of drugs a dentist can prescribe.