American in North Yorkshire, here. It was frustrating to move here originally, struggled with the NY accent and slang - and I love learning about that sort of thing.
However, if the slang is incorporated well, I think it could add, not detract. Like others have said - if the context makes the meaning clear, that could be fun to read. I have struggled through some books with difficult dialect, but at the same time, I hate it when they Americanize UK lit.
I don't like dictionaries in novels if I am forced to refer to it constantly just to understand the story, it takes me out of the story. I prefer to work out the slang from context. I will slog through the slang-dictionary back-and-forth for a very good story, but I don't know if I represent the average reader in this.