Actually, it doesn't (and indeed cannot) use full postcodes at all -- what it requires is the first part of the postcode (W1, E17 etc) plus a road name (or name of a station or sometimes a famous building like Big Ben). It's a shame it can't use postcodes because a lot of people know their postcode but not how to spell the name of their road. On the other hand, lots of people, especially those involved in car accidents have no idea which postcode area they are in and it is a nightmare trying to work it out for them.