Jude,
First, a very warm welcome to Rootschat.
The Sir Edmund Nagle that "served" the Prince Regent was a distinguished British Admiral. He became a Lieutenant in the Navy in 1777, climbing through the ranks. In 1815 he was appointed to take command of the newly built Prince Regent's royal yacht "Prince George". He frequently accompanied the Prince Regent on social occasions.
When he died, March 1830, age 73, he was described as "Admiral Sir Edmund Nagle, K.C.B., one of the Grooms of his Majesty's Bedchamber" (The Examiner, 21 March 1830).
Struggling to find any reference to Sir Edmund's link, whilst in the Navy, to the Sussex Sea Fencibles. But there is very little out there anyway about this force.
What's the source of the "valet/butler" story?