I thought the aircraft callsign was originally the radio station designator ? i.e. the radio callsign assigned to the radio equipment mounted in the aircraft ?
What callsign did Orville Wright use ?
All Radio Hams used to have a callsign that started with G in the UK, and 9V in Singapore, VH in Australia, ZK in New Zealand, etc. and the aircraft now follow suit, but I don't doubt some other bureacracy has taken over, and the CAA will assign 'private' call-signs according to the protocols already mentioned, in the manner of personalised number plates on cars, I guess.
There is also a sort of system for ICAO airfield designators. When Pontius was a pilot someone drew a grid over the world and lettered the squares A B C D E F G etc. most of Northern Europe, including the UK, fall in the E square, the second letter designates the Country, and the last two letters have some sort of affinity with the airfield name - sometimes.
So ... EGLL is the haphazard E, then G for Great Britain and LL for London Heathrow ( why not LH ? dunno ) or EGKK for Gatwick. EHAM isn't Hamburg, but haphazard E, then H for Holland and AM for Amsterdam.EDDF E plus D for Deutschland, and DF for Frankfurt. EGPK = Prestwick.
France and Italy fell into the L square, so LFPO = haphazard L then F for France and PO for Paris Orly, LIRF L + I for Italy, RF for Rome Fumicino.
It's not definitive World wide now, for instance Australia has changed to starting with A, and New Zealand with NZ, which makes sense, but the USA has stuck with the initial letter, most starting with K KJFK for Kennedy, ( used to be KIDL ewild ) but P for the Pacific places, like PANC for Anchorage and PHNL for Honolulu.
Some sort of logic to the system I gues,tho' one does sometimes wonder !