In British English, surgeons, at least male ones, use the title Mr and not Dr, although this distinction does not exist in American English. Generally Hong Kong English follows American usage on this point, even if the surgeon was trained in England. Traditionally surgeons are Mr and not addressed as Dr unless they have a Ph.D. Someone might finish medical school, earn the title Dr, take additional training as a surgeon and then revert to the title Mr to show off their new elite status.
The reason for this use of Mr has to do with the history of traditional Western medicine. Yes, there was such a thing as traditional Western medicine and it was horrid! A fever might be treated by the doctor stabbing the patient to let out "extra" blood. Alternatively parasitic leeches might be used to suck out the patient’s blood. Then the poor patient has to recover from a fever and anaemia (lack of blood – anemia US spelling).
Doctors practicing traditional Western medicine would advise women to eat very little during pregnancy for fear of birth defects. The idea was that if a pregnant woman ate beef the baby might look like an ox, if she ate fish the baby might have another deformity. Anything else she might eat was thought to cause other problems. Pregnant women who consulted these doctors were malnourished from a lack of food. The doctors of course did not actually examine the woman, but only offered theories. That was traditional Western medicine. Surgeons, who had not studied these strange medical theories, were of the opinion that pregnant women should eat more if they were hungry.
It is a common error in Hong Kong to talk about "Western medicine" as compared to "traditional Chinese medicine". Instead they are comparing modern medicine and "traditional Chinese medicine". Modern medicine is not specifically Western and many Asians have contributed their own scientific medical research. Modern medicine is very different from traditional Western medicine.
Long ago, in Medieval Europe, medical doctors had university degrees and surgeons had no formal academic training. However traditional Western medicine was so poor that you were better off with a surgeon. Doctors were educated gentlemen who discussed strange theories. Sometimes they bought their doctoral degree without doing anything undignified such as studying or going to classes. They could use the title Doctor.
Surgeons were tradesmen who had to solve your problem if they wanted to be paid. They would cut your hair, remove a bad tooth and, if needed, amputate a leg. They could use the title Mister (Mr) as the master of a craft, but not the title Doctor. Medicine greatly improved when the Royal College of Surgeons of London was founded in 1800. After passing the required exams, surgeons could put MRCS (Member Royal College of Surgeons) after their name. Today UK surgeons keep the title Mr out of pride to distinguish themselves from ordinary doctors who don't have the same advanced skills.