THE HISTORY AND TRADITION BEHIND THE USE OF THE TITLES OF MALE AND FEMALE SURGEONS 

In the United Kingdom, the Republic of Ireland and in some Commonwealth countries (such as South Africa, New Zealand and some states of Australia), many surgeons use the title Mr (or Miss, Ms, Mrs, as appropriate), rather than Dr (Doctor). Until the 19th century, earning a medical degree was not required to become a qualified surgeon. Hence, the modern practice of reverting from Dr to Mr after successfully completing qualifying exams in surgery (e.g., Membership of the Royal College of Surgeons or the Royal Australasian College of Surgeons) is a historical reference to the origins of surgery in the United Kingdom as non-medically qualified barber surgeons.

Wikipedia