ICAO has divided the globe into zones, and given each zone it`s own letter. Continental USA is a zone all of its own, and has the identifier K. Same for Canada, here the letter is C. In those two countries you just add the IATA designator after the K resp. C and you have the ICAO designator.
England is in zone E, thus the first letter in the identifier is E, then the country has been assigned the letter G, and the last two letters are specific to the aerodrome in question.
Some claim that E stands for Europe, but that`s not the case. Try telling that to the French, they are in zone L....
Here`s a few others in zone E:
EN - Norway
ES - Sweden
EF - Finland
EP - Poland
ET/ED - Germany
EK - Denmark
EH - Holland
EB - Belgium
EI - Eire
Rgds,
AS