The post office will attempt to deliver if you put an address on it.

There is guy living off grid in I believe Dorset on YouTube called "Maximus Ironthumper". The post office told him to try sending himself letters, eventually they started turning up. Then that became the address.

He has a whole series of videos about how he kinda managed to setup his off grid living situation, there is everything from how to avoid planning permission, to how he setup his solar power.