What do you call the particular accent the secondary characters have? I am looking for the right word that refers to the accent of the secondary characters, they have what I would call a "peasant" accent, but I doubt that's what it's referred to by the people who are in the voice-acting community. I am not talking about the Witcher's accent, because he has a normal American English accent.
No, it's not a "peasant" accent. It's seems to be some variation on the British Cockney accent, which is based in London. It's used as a relatively standard "working class" accent. I'm not enough of an expert to say for sure if it's a particular named variation, and it lacks some of the vocalizations which appear in that dialect, as well as some of the its more famous slang terms.
I'm not sure why the game producers have the other characters speak in British accents at all, although it's probably just because it sounds nice. As a contrast, Yennefer speaks in a cultured, upper-class British accent. If they wanted to make these lower-class characters sound like peasants they might have used one of the more rural British accents, such as those from the middle or west of England.
In Game of Thrones, the characters from Winterfell all speak in a Northern English accent, to indicate remoteness from the main city (King's Landing) -- with Sansa Stark as a notable exception, possibly to indicate she was specially brought up to sound more like a "lady".
Here's a rough overview of some of the more well-defined British accents.