Step one: Don't fucking say words
I can't even say I'm "lighting up a fag"? What's the point of even visiting the UK if you can't say that?
Say it in the UK and they'll tell you to enjoy your cigarette. Say it in the USA and they'll call the FBI and report a hate crime about to be committed.
Or that might gather up a flashmob of homophobes with torches and pitchforks (Where did they get those?) ready to set fire on any sodomites they find.
Given that the UK does still get a lot of movies and TV from over here, it wouldn't surprise me if an American saying that was still looked at askance. Every Brit I've talked to (albeit online) has known at least THAT much American slang.
The majority of British people know the American homophobic meaning of the word fag.
I was on this course once, and it was during the break and almost everyone else had gone outside for a smoke.
Then this gay guy came in who I was sort of friends with and asked me where everyone had gone, and I just said "Oh, they've all gone for their fag break." And he just went mental on me "You shouldn't use that word!" "It's homophobic."
It was obvious from the context thought what I meant though, and he was British, so he must have known the cigarette meaning of the word fag.
So yeah, some people may just assume you are being homophobic if you come over here and start throwing the word fag around.
