In many Hong Kong schools, teachers say that they teach British English. I have to hold my tongue and not ask: Which one? There are always different opinions about the English language. There is no official standard that everyone will accept. However when teaching we must not correct each other in front of the students. That would only confuse the students even more. We talk about such issues privately. However in the case of the myth of British English I usually say nothing at all.
There is no standard British English accent that everyone in the UK uses. Instead there are many local variations each with their own historical roots. People even disagree on which are separate local varieties, but there are up to 30 or 40 local varieties in the British Isles. Some of the most commonly recognised British English accents include: RP, Cockney, Brummie, Yorkshire, Estuary, Geordie, Welsh and Scots.
RP (Received Pronunciation) is the variety of the southern upper class and the original BBC. Some people think of RP as standard English, but only about 3% of British people speak it. Cockney is the dialect of eastern London and is famously difficult for foreigners to understand. Yorkshire English and Brummie still use some older words. Estuary English is spoken in London and south-eastern Britain. It is the most widespread and is more closely related to varieties spoken overseas including American English. Geordie is the dialect of Newcastle-Upon-Tyne and is related to the other varieties of Northeast-England. Welsh English is used in Wales. Scottish English is a number of varieties related to the Old Scots language. Scotland has more English varieties than England.
There is a more-or-less standard English grammar used across Britain and in America although there is no one single standard accent. (For a comical example, see the Ascot scene in My Fair Lady, where Eliza speaks Cockney Dialect grammar in an RP accent.)
In class it is best to try to teach the most commonly used form instead of trying to use a standard British English that does not exist. A better approach is that when two competing forms exist, such as the spellings of color/colour, the most common UK form can be the default choice.