Many buildings in London actually have multiple postcodes. In general UK postcodes are granular enough that almost any combination of a name and code should make it, save for situations like several people with identical names living in flats that are fewer than N floors apart.
Postcodes are designed to facilitate sorting and last mile delivery so no surprise if large buildings have multiple codes. DVLA in Swansea has several, probably because they get a lot of post in a few distinct categories.
I'll bet "Rishi Sunak" would get to 10 Downing Street.
I wonder if random mail sent to top level politicians actually gets to them. The intention is clear, yes, but there's probably layers of security that will prevent it from getting to its intended address...
Almost all paper mail received by politicians in Canada is read by their staffers. You'll get a form reply, at a minimum. Some actually do read a lot of it. Whether curated by staffers, or a random selection, or both. I imagine it depends on the individual politician. Some do reply.
R B Bennett, Canadian PM from 1930 - 1935, was an eager correspondent with random people across the country. He read and wrote tens of thousands of letters throughout his career. During the Depression he would include cash from his personal wealth in the envelopes if people described hardships. I wonder if that would be seen as vote-buying today but it was probably genuine charity.
I don't believe that basic tendency has really changed. Some politicians do genuinely want to be close to the people and random conversations and letters with ordinary people are one way to do that. And seeming approachable is useful politically, if nothing else. Some want nothing to do with the filthy masses, of course.
For mail to the U.S. president, it goes to a facility near the White House where it’s screened, sorted, replies are sent (usually selected from a set of pre-generated forms) and (depending on the incumbent) a handful are brought to the president’s personal attention.
If it's a reasonable letter, I doubt the way it's addressed makes any difference.
Members of Parliament have a duty to listen and respond to their constituents, which in Sunak's case is the people of Richmond, North Yorkshire. I don't live there, but I have lived in a senior government minister's constituency in the past. I wrote one letter, and I did get a reply — I doubt my letter was directly read by the minister, but it was probably tallied up by an assistant "15 letters supporting this so far, 8 against".
The difficulty these days is the volume. Rory Stewart (an ex-MP) mentioned on his podcast recently that one of his predecessors in his seat in IIRC the 1950s got about 5 letters a week to deal with; when Rory was an MP in the 2010s he was getting over 20,000 emails a year. It's much easier to have a personal touch in correspondence at 5 a week!
That's about 55 per day, or a little over 100 per working day. Perhaps too many for a detailed personal response for each, but certainly enough to read. Categorising well for future action, response, ongoing follow-up, a part of a staff member's job.
Perhaps, but how much time do we want MPs to devote to reading correspondence versus all the other aspects of their job, such as holding constituency surgeries, reading up on legislation, doing ministerial jobs (which Stewart was at this time), and so on? Especially when a significant fraction of the incoming emails will be the result of some campaign group or another having encouraged its supporters "write to your MP" with a template letter with set of suggested arguments...