Unfortunately there is no current way to change your Spotify username. If you connect with facebook your real name is displayed instead and you can login with your fb email instead though.
About the postcode; that's odd, I suggest you submit an online contact form; and someone at Spotify can look into it for you.
If you get an automated reply; make sure to reply to it directly (even if it's from a no reply); and it will be sent to the Spotify staff.
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Unfortunately it is not possible to change Spotify usernames due to how playlists are mapped to accounts.
As for your post code issue, I would suggest dropping the Spotify team a message using the online contact form and they will be able to investigate that for you. If you get an automated email reply, just be sure to reply directly to it and your case will reach an agent.