Sorry for the slow reply.

Personally, I'd say it is definitely more a "addr:housenumber", than "addr:unit". It _is_ a subdivision of the building, but principally it is a number in the ordered street sequence.

I'd also use "addr:housename" over "addr:substreet".

How it is rendered on an envelope is irrelevant, in my opinion. We are recording raw data which can be used in a number of ways. Unless we think that the envelope is the true expression of the nature of the address data and that's what we record ­— and I don't think that's the case.
In general, I am not a fan of that wiki page about UK addresses, so take this as you will.

All the best.