I get my information from two sources:-
http://www.wolfbane.com/vreg.htm
https://www.mycarcheck.com/
The latter sometimes needs the cookies to be cleared in order to continue 'asking' - though by 'backspacing' from a successful search you can usually start again without problems.
My basic recognition of the original registration letters is something that I was indoctrinated in by my father from as long as I can remember.
Of course most of these are regional and restricted to the North East, though the obvious inclusion of the letter S indicated Scotland and, when I was at grammar school I 'learned' the registrations of the cars used by the teachers (though I'm not always able to recall these nowadays).
At one stage the AA members handbook included a table of registration locations.
I contribute the registrations so that other members can share the information (and maybe begin to learn these letters for themselves).