Hi again, Paolo --
Best wishes to you as you continue to search for answers. When you find a doctor, be sure to go prepared with your questions written down...sometimes it's hard to remember everything when you're in an unusual environment like a doctor's office. Particularly, the stomach pains you described in your other thread are troubling. Neither reflux nor a hiatal hernia will cause stomach pains.
General procedure for an endoscopy is: "...nothing to eat or drink past midnight." Call the medical center to double-check.
This couldn't be more of a shot-in-the-dark, but you may try contacting David Jones for specific questions about opera singers, reflux issues, and perhaps doctor recommendations. He is one of the best classical voice instructors in the world, and used to maintain a London office. (Don't know if he still does, but annually does training clinics there.) His website is here: http://www.voiceteacher.com/
I think you might enjoy reading articles from his archive, found about halfway down the left-hand column of the home page.
The only reflux doctor I've heard of in the UK is Dr. Ken Park, because of his work with the Linx. Not trying to steer you in that direction, but the fact that he is trained in this procedure perhaps indicates a greater understanding of GERD and the persistent symptoms that other therapies have difficulty resolving. Here is an interview: http://www.youtube.com/watch?v=-F4SxZx8T7c At any rate, he is a surgeon and maybe not the best for a consultation, but his staff could direct you to a GI he works with.
I mentioned Pepto Bismol in your other thread, and I think you said you hadn't heard of it. I'm fairly certain it should be available for sale at your local pharmacy. It might be helpful to try it for your stomach pains before you see your doctor, as a cheap diagnostic tool. If it helps, then that may indicate a direction for the doctor to go. If it doesn't help, then it eliminates some possibilities.
Best wishes.
-Bruce