"British people speak English."
2020.5.28 I have heard 講 jiang3 used for "to speak and talk" in informal situations in both mainland and Taiwanese dramas, which quite surprised me because I thought they always use 說。 I think some confusion may be that 講 is used in many compound words that mean lecture, speeches, classes etcetera
講演 jiang3yan2 to give a lecture, speech
講師 jiang3shi4 a lecturer
講座 jiang3zuo4 class lectures
講價 jiang3jia4 to haggle over price
講[讲] is quite common as a synonym for the independent verb 說[说], especially in the more southern regions of the Chinese-speaking world – probably because most Southern Chinese languages only use cognates of 講[讲], not 說[说] (in Classical times, 說[说] specifically meant “to explain”, so its cognate in many Southern languages only has that meaning, not “to speak”). So 講英文[讲——] is fine, as is 講話[讲话] etc. In my ears, 講[讲] has a slightly more colloquial ring to it than 說[说], but that may be because I’m used to Taiwan Mandarin.
Also in compound words, the two are not exchangeable: 小說[—说] “novel” is never *小講[小讲], and likewise, 講座[讲座] “lecture” is never *說座[说座].
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 people from America and from India 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.
英国 refers to Great Britain as a whole. If you want to talk about just England without Scotland, Wales and Northern Ireland, that's 英格兰. But in practice such situations are not all that common, and certainly you wouldn't talk about a person as 英格兰人, unless you specifically want to stress that they come from England, not another part of the UK.
The language is 英语, regardless of where the speaker comes from - just like French is 法语, regardless of whether the speaker is from France, from Canada, from Senegal or whatever (or even a second-language speaker from a non-French-speaking country).
Kind of. When used as suffixes for languages, they are mostly interchangeable. If you want to be nitpicky, -ᅟ文 technically refers to the written language, -语 to the spoken one, and that tendency is still there in practice. Nevertheless you do hear -文 used for spoken language and -语 for written one as well, just somewhat less frequently.
Also there is a small selection of language names which only use one or the other, most notably 中文 and 汉语 (both referring to Chinese). 汉文 exists, but it refers to Classical Chinese texts, not to the modern language, while *中语 does not exist at all.