A friend of mine who lived in a tent in a park got his own postcode. True story.

Details: election time. He went to the election folks and asked for his election papers. They said "sure, where do you live?" he said "the Bender, Eastville Park, Bristol", they said "that's not a valid address", he said "that's where I live, so that's where I'd like my registration to be, please". There was some back and forth. They caved, and duly entered his address on the electoral roll as such. Then he went to the Post Office and said "this is my address, as entered on the electoral roll, can I have my postcode please?". The Post Office kinda had no option, since this was now his official address. So they gave him a postcode and the postie had to walk through the park to drop off his mail.