Well, the RN did have HMS Neptune.
I’m not sure, but Post-Captain may be the answer that I’m looking for. According to this site, once one had been put on the Captains List, promotion to Admiral was automatic. BUT men could not be promoted out of the order they were put on the Captains List. So if the admiralty wanted someone 4-5 names down the list to get to admiral, they had to promote the guys senior to him. I guess those senior guys ended up as Admirals of the Yellow.
Plus you get the power to telepathically control fish.
The issue with Rear Admirals in the US Navy is complicated.
First, understand that the USN didn’t have admirals until the Civil War - David Farragut, if I recall, was the first USN at each level: Rear, Vice and Full. Prior to that, the highest rank was Captain, with senior captains holding Commodore billets.
Once we had admirals, the issue was cross-service. The closest USN equivalent to a Brigadier General was Commodore. The permanent ranks were Rear, Vice and Full. USN Captains went straight to Rear Admirals, making every single Rear Admiral rank every Brigadier General. The Commodore/RADML was/is a cross-service compromise to have a USN rank level with BGEN.
And yeah, the old RN promotions were basically by seniority, which sucked mightily for talented officers.
Nelson doesn’t seem to have had the best luck with ranks. He was only made a Baron when they let him into the house of lords. Lowest rank. Did, however, get made a Duke by the Kingdom of the Two Sicilies. He was in command on the fleet in the Mediterranean at the Nile (which got him his Dukedom) and at Trafalgar. He countermanded Hyde Parkers order to withdraw, although the ships attacking the, I believe, Trekroner fort only saw the original order and withdrew, suffering significant damage as a result.
What nobody mentions* is that this was done in the early days of signal flags, when there were no special flag sequences that encoded phrases and everything had to be spelt. Unfortunately, this was also before the advent of modern schooling, so the low-ranking signal boys were illiterate. Hyde Parker, in desperation, screamed orders at the tops of his lungs, “splitting them with blood and thunder,” as a popular account had it, to be heard. Due to his show of tactical genius predicated on yelling loudly enough to be heard from another ship, the British named one of London’s most important landmarks after him: You can still hear people screaming in Hyde Park.
Tom Tildrum: Lord Nelson did indeed only have one arm. This did not stop him from expressing himself musically, or from owning an aurally-challenged big cat. In fact, he named his most important musical composition after this valued pet, which he wrestled to the ground in wildest Africa, and caused mighty stirrings in England when he performed it. This was the era of the gutter press, you understand, when spelling was posh and, therefore, suspect, but enthusiasm was almost as common as the teetering yawls. This enthusiasm created a phrase as true now as it was back then: “Def Leppard’s drummer’s only got one arm!”
*(That would be because it isn’t true. Pity, really.)
I find it hard to believe that signal boys were illiterate. Isn’t this the one job on board the ship where you’d want to employ a literate? 
Well, there were laws against teaching pigs to read, and, since the Corn Laws (passed by Robert Corn, who also organized the Bobbies (which the Irish called the Peelers)) defined all cabin boys as ruminants for the purposes of capitations and poll taxes, they were, sensu stricto and quo warranto, illiterate. This is why “fo’c’s’le” and “bo’s’n” are spelled like that: Illiterate people were charged by the letter, and you couldn’t get much better on a signal boy’s pay.
I see what’s happening here!
That’s impressive, as Lord Nelson only saw half of what happened. In fact, he once missed an important signal because of that.
He was also kind of absent minded - what with putting his spyglass to the wrong eye and all.
Just because I heard it on QI once I thought I’d mention that Nelson originally intended the signal at trafalgar to be “England confides…” ratyher than “England expects”, but there wasn’t a flag for “confides”, so he changed it.