The Yeomen of the Guard are a ceremonial body and have been for hundreds of years. The last English monarch to take the field with his bodyguard was George II at Dettingen in 1743 as you mention above. They are often confused with the Yeomen Warders at the Tower of London (both are sometimes referred to as "Beefeaters") but they are separate groups.
Neither group fielded troops during WWI but as both have always recruited from ex-servicemen it is true that some men who subsequently joined either group were WWI veterans. Again as the quote above explains, during WWI they resumed the guarding of Royal palaces but this was static duty inside the UK and not a specifically "military" role. Whilst it is possible that men of the YoG might have volunteered to serve in WWI but they would have been as members of other regiments / corps. However, as they were already time served veterans (min 22 years and rank of Sgt) I doubt many if any at all would have been eligible due to being too old.
Something that can cause confusion is the the British Army has always had regiments called "Yeomanry". The best way to explain the term "Yeomanry" is to say that they are the cavalry equivalent of the Territorial Force infantry. That is to say they are mounted / armoured units but like the Territorial Force they are not part of the Regular Army. Today Yeomanry units along with all the other non-regular regiments / corps units are simply classified as "reservists".
I hope this helps.
Regards
Mark
The history is too complicated to give a synopsis here but once read and understood it becomes clear that the YoG has not been a "combat" formation for centuries.
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