Re: Post codes
"... As far as I can discern, postcodes are based on a number of dwellings ..."
(this is from memory, I've not used the PO db for a few years)...
it's ... "something like that".
In towns & cities a post-code is often (but not always), a "street", ie a vaguely sensible collection of address that have some sort of connection - obviously real-life is not neat and tidy so this is only really a "guideline" in practise.
Out in the countryside the post-code defined areas (yes, they're geo-spatial areas with boundaries) are a bit more random, so some might contain 20+ dwellings, some more, some much less (our one has 4 dwellings, but 6 addresses (iirc) - because one of them is a farm with multiple out-buildings and businesses onsite). The centroid of the area is located about 100metres from my dwelling, luckily in site of my gate, but if you're at the post-code centreoid you can't see my house! (the gate itself is mostly hidden in hedging).
Also - again, if memory serves, the PO used to licence the geo-information at different granularity levels for different prices, so it was very possible for cheapskate companies to buy lower-granularity information depending on their use case, or parsimoniousness.