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.

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.

I used to work in the correspondence unit of 10 Downing St. Very few letters made it as far as the PM's desk, but all letters received a response.

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".

I wonder if a letter addressed to “Liz Truss” would have got there in time for her to still be in residence.

Don’t know about others but Reagan used to respond to a lot of letters people sent. He complained that it usually took months through the bureaucracy.

Excellent book has a collection of such letters.

“Reagan: A Life In Letters“