In message , John Salmon
writes

As a matter of interest, where did 'IG' come from? In my view, its
derivation is the least obvious of all the postcodes. I can work out
all the other slightly obscure ones e.g. SP=Salisbury Plain,
DG=Dumfries & Galloway etc., but the only suggestion I've ever heard
for IG is Ilford & Gants Hill, which seems unlikely.

I suspect that IG reflects the names of the first two post towns within
the postcode - Ilford (IG1 to IG6) and Chigwell (IG7).

Much the same happens with SM, where the first two post towns within the
postcode are Sutton (SM1 to SM3) and Morden (SM4).

Presumably IL was avoided because OCR equipment couldn't be relied upon
to read the letters accurately, especially when hand written.
--
Paul Terry