(note: this may no longer be true - I learned this in the late 90s, and my recollection is slightly fuzzy, so take the details with a pinch of salt).
Royal Mail postcodes that people commonly use may be augmented with a so-called Delivery Point Suffix (DPS), that is an extension of the postcode that identifies a unique delivery address.
The DPS is an alphanumeric code like "1A". As such it is likely shorter than the decimal property number, and does not waste characters on things like "flat 2".
I think a postcode plus DPS should be sufficient to get a letter to a specific address.
High volume bulk mailers can qualify for discounts if they use the DPS, and apply sorting to bag up mail based on destination sorting office and the like.
Sometimes you can see the DPS printed after the postcode on your bills if you get mail from a company that has huge mail volumes.
I learned all this when I re-wrote a god-awful bulk mail sorting program called qamsort (Quick Address Mailsort) for a mobile Telco billing department. Back then Royal Mail used to give away the data needed to do the sorting. Took me a week of lunchtimes, written in Perl, ran in a fraction of the time of qamsort, and didn't cause monthly callouts at 2a.m. because of bullshit license key file expiry.
Never got used in production because people were too change averse and my boss didn't think we could sell it for enough to be worth taking on the risk, which turned out to be the right decision because Royal Mail started charging a pretty penny for access to the sorting data not long thereafter.
I was very excited to learn Perl back then. Fun times.
Classic.