Thank you, Andrew. One of the reasons I asked was because the only man to have genuinely risen from private to field marshal in the British Army, William Robertson (whose mother disowned him with the words "I would rather bury you than see you in a red coat"!), was a cavalry trooper, who so impressed his officers that they had a whip round and paid for him to go through Sandhurst. Colin Campbell also did it, but was a Volunteer (9th Foot originally, I think) having been mistaken by the Duke of York as a relative of someone to whom the Duke owed a favour.

The riding instructor point was interesting; during the AWI, sergeants from the 16th and 17th LD were seconded to Loyalist cavalry corps (primarily British Legion and Queen's Rangers, but others as well), and presumably must have instructed both officers and R&F, but were all returned to their units, and I'm not aware of any of them being promoted to commissioned rank (certainly not as a result of that particular appointment). I did wonder if the French Revolution and Napoleonic Wars saw an influx of "foreign" riding instructors (in the same way that foreign musicians were hired as bandmasters), in order to improve the standard of horsemanship, as per Le Marchant.