I have sent something to "{name} Somewhere near the brewery {town}" and it reached the destination. They will often try and deliver these kind of incomplete addresses.

Many postal systems have a "difficult cases" department whose job is to try to decipher otherwise undeliverable addresses.

In the UK the postcode would be often applied to only one street (or a part of a street). This means in practice you get small villages with a postcode that covers 2 houses for example. The local postman could likely know the residents well enough in that case.

SW1A 1AA is sufficient for a letter to Buckingham Palace. You should probably include the name of the person.

It's not unusual. The university residence halls I lived in had postcodes unique to the building, and my first employer in London has several postcodes for several nearby buildings.

Average number of properties per post code is 15 (max about 100), so a lot smaller than a route. And in rural area the routes are often kept to the same small group of staff for years.