I’ve always been puzzled how these work. Even aircraft from the same squadron don’t seem to have common markings. I thought the pilot’s name (initials) were part of the code, but it seems not. If I wanted to include two (or more) fighters from the same squadron what would the markings typically look like?
From the very little research I’ve done, this is hard with the RAF, particularly early in the war. Planes were scarce and they got transferred between units without being repainted. Having the right codes wasn’t a priority.
I do know that pilots initials were not part of the code, with the exception of famous group leaders like Doug Bader and A.G. Malan who got permission to do that.
Squadron codes were introduced around the time of the Battle of France in spring 1940. A two letter identifier for the squadron, and a single letter identifier for the aircraft itself. Prior to this, many aircraft simply had a single letter identifier for the aircraft itself, and some squadrons wore some sort of insignias. But for intelligence purposes, those insignia were usually removed. Also command pennants would be painted usually just below and ahead of the windscreen.
Later on as the war progressed, squadron, wing, or group leaders began to sport their personal initials on their assigned aircraft. Also later on in the war, some squadrons used a two character letter number combination instead of two letters. Squadrons could also change code letters if they changed theaters of operations.
Still sounds complicated! 
So…I might have "XY (squadron) - roundel - A (a/c identifier), for 1940? 
Not really. For example, Ian Gleed flew a Hurricane I coded LK A. LK was the squadron ID for 87 squadron, and A for the aircraft, a Hurricane I serial P2798. Later he was promoted to squadron leader and that pennant was added onto the fuselage near the windscreen on that aircraft, but still retained the LK A codes.
Later, when he became a Wing Commander, he began to use his personal initials, IR G, along with the command pennant and his personal nose art on the Spitfires that he flew while holding that position.
This marking system was adopted by the USAAF 8th and 9th Air Forces while operating over Europe from 1943 on.
And yes, you could have that coding as you have listed it.
Easiest way is to decide which aircraft and squadron you wish to model and then look up the codes. There are lists available on line. For the “aces”, like Johnnie Johnson (JE J), Douglas Bader (D * B), as they reached higher rank they could code their aircraft personally, as the others have said, but even quite famous pilots didn’t do this. Perhaps one of the most famous Fighter Command aircraft is Spitire KLB flown by Al Deere, which was the subject of the first Airfix 1/24th kit. Another is DW*D from 610 “County of Chester” Squadron, Auxilary Air Force. The codes also covered bomber squadrons and other Commands, including Coastal Command, so for example 617 “Dambusters” were coded AJ, Guy Gibson’s aircraft was AJ * G.
Thanks…Thanks…and Thanks! 
Here is the Wiki article with the list of squadrons and code identifiers