VS of warships of the Royal Navy Home Fleet setting sail from Portland Harbour for exercises. According to the old records ships include Battleship HMS King George V, Cruiser Birmingham, Destroyers Myng, Zest, Zenith, Zambesi, and Zodiac and the Submarine Seneschal.
Only some, notably the "T" to "Z" classes, those that mainly finished up as type 15 and 16 frigates, and the first group Battle class. Second group Battle class, and the Darings wore "I" as their pennant, Weapon class wore "G", Hunts type 1-3 and first two type 4 wore "L", remainder of type 4 wore "H", also the ex French and Dutch destroyers. There didn't seem to be any rhyme or reason to it, but no doubt someone wiser than I will be able to explain. My information only comes from a series of little Ian Allan paperbacks, this particular one being "Warships of World War 2, part 2, Destroyers and Submarines".
I've just found a paragraph on pennant numbers which makes it even more confusing----------------as follows.
"At the outbreak of war destroyers had flags D,H,or F superior to their numeral pennants. The Tribal class were originally alloted flag I, but this was changed to flag F in december 1938, and flag L used for escort destroyers building (Hunt class) or converting (V and W classes). In 1940 flags D & F were changed to I & G and war construction was allotted the latter letter until the T class which adopted flag R (see note). A reversion to flag G was made with the Weapon and cancelled G classes, and as war losses had left many gaps in the original lists the later Battle and D classes reverted to flag I. Post war, flag D was allocated to all destroyers and in consequence some re-numbering was required: usually by the addition of 100 or 200 to original numbers
Note:- The R class originally had flag H but this was changed to flag G soon afterwards."
That is a verbatim copy, and if you can make head or tail of it you're a better man than I am Gunga Din. I'm now going for a drink to stop my head spinning.