Originally Posted by Jenbel
B747-437B, that seems logical, but none of the UK codes' last two letters designate the local aerodrome. I can't think why LL would be LHR, BD - EDI or NWI is SH - it's not even releated to other names (e.g. EDI - Turnhouse). Might be used in some countries, but not in all of them. Could this have been left to local aviation authorities to decide how to code the aerodromes?
As explained by WHBM, the UK allocates its airport codes on a regional basis (as does France, BTW)
There are some exceptions to the rule - for example, the now moribund Sheffield CitY requested (and was given) EGSY... a code which had, only a year previously, been allocated to Fowlmere, a small private airfield near Duxford (EGSU). Fowlmere became EGMA instead. (The EGMs are more commonly found around South Essex and Kent - Southend is EGMC, Lydd is EGMD). Also, some airfields have the "wrong" third letter because everything in the right block had already been allocated - I believe that's how Little Gransden (another Cambridgeshire airfield) ended up being EGMJ. There were no EGSs left.
Errr... yes, real anorak stuff (do our North American cousins understand the term "anorak" in this contect?)
Note: Heavily edited after original submissions because I realised I had repeated some of WHBM's posting. Not quite awake yet!!