What is the minimal possible UK address?
In the US, someone I know who had a Post Office box liked writing simply `box-number zip-code` as their address.It worked.  Eg, if you wrote on an envelope:    1034
    21240

It would get there!  You might want to add a "Box" before the "1034" to be safe -- and to disambiguate between a house number, as in OP? (I'm not sure just a house number would ever work in the USA?)But it looks cooler with just two numbers. Especially it did 20 years ago when my friend did it. Very futuristic cyberpunk.  Maybe a square glyph for "box" would be good.