I always write the ICAO codes on the basis that if I can be bothered to give the information then the authorities can jolly well train their staff to know which airports they refer to.
. For the same reason I always give zulu time.
To be fair if you follow the Vapours site we will apparently be able to do it all on line soon, but if it is anything like self assessment that will make the process far more complicated contrary to what you might expect - me, cynical? Ah well, it is all progress apparently.