Learn English – Is “Her hair is in a pigtail” (singular) used in British English

Is “in one go” British English or just English?

I've wrestled with this one, mostly because of the way you've titled your question.

Take, for example, nappies (which we call diapers in the U.S.). I would consider nappies to be UK English; I rarely hear the word, and, more importantly, when I do, I almost have to translate it in my mind.

As for in one go, I looked at a lot of blogs and message boards, and, indeed, when I managed to find this expression on the internet, it was almost invariably traced to a U.K. speaker¹. But, for some reason, it doesn't sound chiefly British to me. It's immediately understandable. I got this done in one go doesn't sound like something I would never say, (unlike, I need to go change a nappy).

So, getting back to your title, I find myself wondering: What makes something "British English" vs. "just English"?

If I had to make a ruling, I'd say, no, "in one go" is not "British English", and I'll count on Macmillan to back me up.

Here is the definition of nappy in the American version of Macmillan:

Now here is the definition of the noun go in the same edition:

So, the Macmillan editors, at least, don't seem to think the phrase is British enough to be tagged BRITISH.

That absence of a BRITISH tag doesn't appear to be an oversight, either; that same entry also reveals:

In short, Macmillan would categorize

I'll have a go at answering this question

as British English, but

I typed this whole answer in just one go

would be what you called "just English."

Based on my usage searches, though, I think it's a borderline call, so I wouldn't vehemently argue against Codeswitcher's stance.

¹Like this one, from an electrician:

In the latter case, then you'd be doing yourself a favour to have the whole lot done in one go.

The speaker is a self-identified electrician from Thornbury, which I assume is a U.K. Thornbury, judging by the way favour is spelled.

The flap (which is written as "ɾ" in the IPA) is an allophone of both "d" and "t" in North America, among other places. An allophone is one of the continuum of sounds that we perceive as a certain "ideal" phoneme, a deviation from this ideal. There will be different extents of deviation depending on the speed with which one is talking, regional differences (e. g. Californians would be more guilty of flapping than someone from New York or Kansas), formality of speech, etc.

I, for example, am hearing a distinct "d" in "according" in this video (0:38) and, e. g. in "heard and" in this video (3:39)

So, there will certainly be more flapping in AmE generally compared to RP in BrE, but it's not like you'll suddenly and definitely be outed as a foreigner as soon as you sound one good "d" in these positions.