"British people speak English."
Another way that is more common in Taiwan is to say languages like so:
Chinese: Zhong1 Wén English: Yīng Wèn,
but for English, the Y is silent so it sounds more like 'ing'.
(Also by 1 I mean the first tone)
According to this online English-Chinese dictionary entry for 英国人, yes: http://www.mandarintools.com/cgi-bin/wordlook.pl (you might have to enter "British,” “British person" or "Britain" in the search box). Then again, China is not the only place where the terms "English" and "British" are used interchangeably, especially in a casual, imprecise way that is technically not always correct: I have heard Americans confounding the terms as well (and, for that matter, using the terms "England," "Great Britain," and "United Kingdom" interchangeably and often incorrectly).
If you want to specify, I believe you can say 不列颠诸岛 (bu2 lie4 dian1 zhu1 dao3) for "British Isles" (and 不列颠 for "Britain.")
Maybe the English word "Chinese" makes a similar confusion; e.g., in English, the word "Chinese" can refer to "Mandarin," "Cantonese," or any other Chinese languages.
what's the difference between 英语 and 英文? or is there no difference at all and it's just whatever feels right in a given moment?
I have heard that 语 is more often used for the spoken language and 文 for the written language, yet, I have heard both said either way. Actually, I have heard 英文 more often, but that might just be the personal preference of the people I have happened to speak with on a regular basis, or, maybe they have been accommodating me: perhaps I say 英文 when I should say 英语, but rather than pick at the error, they "humor me" by using the word 英文 themselves, because they realize that's the term I know and am comfortable with. It's a possibility; maybe I'll ask. If I do, and find out, I'll come back with an edit.
"Barbaric"? Like something Mongolians would say, full of awe, while attempting to invade the Middle Kingdom? Please do explain.