When Lady Edith visits a Harley Street doctor his nameplate is shown outside his surgery. His name is prefixed with "Dr." However, one of his post-nominal qualifications is "FRCS." As a Fellow of the Royal College of Surgeons, his name should be prefixed with "Mr.", as with all qualified surgeons in the British Isles.
Set in 1924, a set of publicity shots of the Series 5 cast members, new and old, were rolled out, and one shot in particular was quite revealing. Sitting on the mantelpiece behind cast members Hugh Bonneville and Laura Carmichael, who play the Earl of Grantham and Lady Edith, was a modern-day plastic bottle of water.