…to be honest I wasn’t even aware that it was British (I’m L3 but I use RP as reference). I picked it because it sounded a bit less blunt than “fuck off”, but not much.
In other words although I’m not a native English speaker (L3 = third language) I tend to use the language as some British people do. As such I might end using terms that are mostly restricted to the UK without noticing that they may not be understood by others.
That’s the case with “sod”. It’s something like “damn” or “fuck”. Sometimes also used as a noun to convey contempt towards someone, as in “that sod” = “that fucker”, “that poor thing”. And “sod off” = “fuck off”, but it feels a bit milder.
[RP = received pronunciation. I used it slightly off-context here but it’s just a reference anyway.]