>blogsis updated the site in response to my dumb nitpickReading it though, I'm afraid you've got it backwards. Anna moved FROM Sapporo, (the capital of Hokkaido and the London analogue), TO a small seaside village. The movie starts in Sapporo.
Despite various English-language sites claiming otherwise, Kissakibetsu is not the name of the village, but merely the closest station. They probably just saw Kissakibetsu in English and assumed that was the town even though they drove from Kissakibetsu to get there, describing the place they're heading as much quieter but not referring to it by name.
Anna goes to 岸浦 post office to post her letter before her first Marsh House trip. That name is never spoken in the film and exists in the real world as a family name read Kishiura. That said, across all of the setting material I've seen online or in my collection of promotional material and the BD the film is simply said to be set in Hokkaido in a village by the sea, with no mention of the village's name. The non-specificity is probably how you got mixed up originally.
It's probably Kishiura, but they definitely took no care to establish this as part of the setting. Maybe a novelisation or some of the BD extras would talk about it more. Incidentally, I came across a fan novel where the writer used it as the name of the town
https://www.pixiv.net/novel/series/567795 
