So if BXA is Willesden,where is BXHV ?
National Identity Cards were issued based on the "census" taken 29 Sep 1939. The registration number on the card was of the format ABCD/12/3, where ABCD is the enumeration district code, 12 is the schedule number, and 3 corresponds to the 3rd person in that household.
I'm guessing that post-war they started using the 3 digits district codes, though I could not find a list of the 4 digit district codes used in the National Registration census (there were 65,000 enumeration districts). Presumably BXHV was an area in or near Willesden.
Alexander