The Royal College of Surgeons of England gives the following account:
Why are surgeons in the UK called Mr/Miss/Ms/Mrs, rather than Dr?
In most other parts of the world all medical practitioners, physicians and surgeons alike, are referred to as Dr while in the UK surgeons are usually referred to as Mr/Miss/Ms/Mrs. This is because, from the Middle Ages physicians had to embark on formal university training to gain possession of a degree in medicine before they could enter practice. The possession of this degree, a doctorate, entitled them to the title of ‘Doctor of Medicine’ or Doctor.
The training of surgeons until the mid-19th century was different. They did not have to go to university to gain a degree; instead they usually served as an apprentice to a surgeon. Afterwards they took an examination. In London, after 1745, this was conducted by the Surgeons' Company and after 1800 by The Royal College of Surgeons. If successful they were awarded a diploma, not a degree, therefore they were unable to call themselves 'Doctor', and stayed instead with the title 'Mr'.
Outside London and in the largest cities, the surgeon served as an apprentice like many other tradesmen, but did not necessarily take any examination. Today all medical practitioners, whether physicians or surgeons have to undertake training at medical school to obtain a qualifying degree. Thereafter a further period of postgraduate study and training through junior posts is required before full consultant surgeon status is achieved. Thus the tradition of a surgeon being referred to as Mr/Miss/Ms/Mrs has continued, meaning that in effect a person starts as Mr/Miss/Ms/Mrs, becomes a Dr and then goes back to being a Mr/Miss/Ms/Mrs again!