Have UK English as well as American English? It's 'Grey' not 'Gray'.
When you teach English, you have to choose which version you teach. Duolingo, based in the US, obviously chose US English. This should only concern the questions and the official correct answers. Correct variants from other major forms of English (definitely UK, probably also Canadian, Australian) should normally be accepted as correct answers. Not right from the beginning, but increasingly, as these variants are reported.
As the differences aren't dramatic, the US English course is quite suitable for learning UK English as well. Sooner or later they would have to learn about US English anyway. Therefore, don't expect a UK English course before we have the much needed courses for other major languages such as Russian or Chinese.
No doubt you can explain precisely why it is that you disagree with mainstream linguism on the existence of British English, aka UK English. And in what respect you disagree. Are you claiming that people in Scotland and in Northern Ireland speak precisely the same variant of English that people in England typically refer to as 'English English'? Or maybe you are even claiming that the differences between British English (or more generally Commonwealth English) and American English are negligible? Or are you conversely claiming that only the English spoken in England deserves that name, whereas other variants should be referred to as Scottish, American, Australian etc.? With all those bizarre forms of regional nationalism going on in the UK at the moment, it's really hard to guess where someone making strange claims is coming from, and what they mean to say, if they don't volunteer a bit more information.
There is a clue in the country's name. All other forms need modifiers, but English, as spoken in England, does not. Even within England there are multiple accents and dialects, but common elements of grammar and syntax are understood by all.
"Commonwealth English" is an even more risible concept. Not quite as bad as Microsoft using "International English", but equal fallacious.
So you are claiming that the variants of English spoken in different parts of the UK don't share enough features that set them apart from other variants to warrant an umbrella term for them? And at the same time you claim that a single word for the variants of English spoken in England is warranted because "common elements of grammar and syntax are understood by all", something that actually holds for all major variants of English?
Sorry, but I have come to the conclusion that trying to discuss with you about linguistics is a waste of time as you either aren't even trying to think and argue scientifically or aren't able to express your thoughts in English.
Perhaps you need to study English more - George Bernard Shaw is the one who described the USA and the UK and two nations divided by a common language (use the word "rubber" as a splendid illustration for the scope for confusion if not familiar with the vagaries of American English).
But the point is very straightforward. In ENGland we speak ENGlish - not UK English.
I make no value judgement here: American English, Australian English, New Zealand English, Indian English, East African English, South African English an Canadian English are all perfectly valid cousins (none of which are Commonwealth English), all with their own little traps waiting to snare the unwary.
But when referring to ENGland it is simple ENGlish.
No-one, in order to distinguish French as spoken in France from Quebecois, would refer to it as French French. It is the default.