British Royal Marines

I am looking for someone to protect a servant (Protagonist) of the Queen while she performs secret work in the Palace. This young woman answers only to Her Majesty. These soldiers must be absolutely reliable. Would that be a British Royal Marine, not a spook?

Would someone be able to give this tale a read for the correctness of British speech, customs, and Royal protocols? At this time I do not think this story will be overly large. Perhaps twenty thousand words? It may be seen as a bit of a parody on the state of the world, and not a criticism of the Royals.

I have no idea why I want this to be British?

Gwen