Mystery Solved!!

Mister or Doctor?
Holders of FRCS (and the new, but not old, Membership - MRCS) qualifications often choose to relinquish their title of "Doctor", reverting to "Mr", "Mrs" or "Miss". This is a relic from times past when surgeons did not attend medical school and were simply skilled tradesmen, amputating limbs or removing bladder stones, and learning their skills through apprenticeship. Note that in 1540, the United Barber Surgeons Company, a tradesmen's guild, was formed by Henry VIII, as many people practised both. In 1745, surgeons were formally separated from barbers by George II.
An alternative explanation is the section of the Hippocratic Oath which runs: "I will prescribe regimens for the good of my patients according to my ability and my judgement and never do harm to anyone," - surgery by its implicit nature does harm to the patient, so the surgeon was not considered worthy of the title "Doctor". Again, this is an ancient practice, that has remained to this day even though it has no real grounding.
"Doing no harm" can be applied to prescribing (by physicians) as much as to operating. Also, many Greek practitioners who followed Hippocrates performed surgery as well as prescribing medicine. The separation was enforced much later by the Catholic Church, through a Council of Tours in 1169 which halted the practice of surgery by clerical physicians by proclaiming, "Ecclesia abhorret a sanguine." This would have relegated the art to the barber-surgeons associated with the monasteries.
Additionally, the change of title is confined to the United Kingdom, and is not practised in other countries where the Hippocratic Oath may be widely espoused.
It is also pertinent to recall that the use of Mr as a courtesy title for all men is a relatively recent invention, and implied a larger degree of status in past years than at present. Compare Esquire.
The practice of surgeons reverting to "Mr" (etc) is obsolete in the rest of the Commonwealth. In the British Isles, holders of an FRCS who move into non-surgical fields tend to go back to being "Dr". In Scotland, only certain surgeons change to "Mr": in Edinburgh ophthalmologists, ENT surgeons and obstetricians & gynaecologists would remain "Dr", but in other cities usage is more like England.

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