An aside: due to the way the UK postcode system works, the 5 digits I'm given are basically equivalent to the 5+4 full postcode nobody uses in the US; because I'm in an apartment building, the postcode identifies the specific building I'm in, which is handy when typing in addresses online... unless I'm not listed as existing.
Finally I send an email to the post office asking if they can maybe investigate why this is.
The top two floors of my building have a separate postcode.
I'm staring at a hand-written post-it I've had on my wall since the day I moved in, written for me when I asked the front desk "My parents want to send me some things, what address am I giving them, can you write it down for me?"
It has the lower-floors postcode.
I've lived here since September. I'm moving out in a little over a month. All this time I've had the wrong postcode. Lolwut. In case it was a once-off mistake, I've asked around. We were ALL given the wrong postcode. Golf claps, people.