So after careful research (watching the sketch over and over), I think that Blenkinsop is supposed to be a Major, and Maltravers and Simon are supposed to be Lieutenants (because a second Lieutenant would really only be there for note taking, even if say, Daddy was an Earl), but I couldn't tell you for sure as you can't see the full sleeve detail.
Also, medal wise he has both the Queen's South Africa Medal and the king's South Africa Medal, which means that he not only served in the Boer War during the King's reign, but also in Victoria's, as to have the King's South Africa medal (thanks Wikipedia!): "The medal was not issued alone but always with the Queen's South Africa Medal or QSA. The KSA was awarded only to those troops who fought in 1902, and who had served for 18 months. Service did not have to be continuous, but even with continuous service the recipient would have had to serve from December 1900 to have 18 months service before the war ended in May 1902 (and commencing before the death of Queen Victoria on 22 January 1901)."
So that's just completely destroyed all head-canon about those two.
Re: Rank insignia
Also, medal wise he has both the Queen's South Africa Medal and the king's South Africa Medal, which means that he not only served in the Boer War during the King's reign, but also in Victoria's, as to have the King's South Africa medal (thanks Wikipedia!): "The medal was not issued alone but always with the Queen's South Africa Medal or QSA. The KSA was awarded only to those troops who fought in 1902, and who had served for 18 months. Service did not have to be continuous, but even with continuous service the recipient would have had to serve from December 1900 to have 18 months service before the war ended in May 1902 (and commencing before the death of Queen Victoria on 22 January 1901)."
So that's just completely destroyed all head-canon about those two.
FFFFFFF.