There is no "original form" still spoken today. Why would the version that happens to be currently socially accepted as "correct" by most native speakers in England be the "original" form? The vast majority of English speakers don't speak British English.

I don't have any good argument why, besides the one that in non-English speaking countries, English == British English.

Same for Spanish, when I'm talking about that language it doesn't even cross my mind that someone might think Mexican Spanish.

But English is not my first language so it may be different e.g. for Americans.

Well ok, but British English is composed at least of Scottish English, Yuppie English, Townie English, Farmers English, Northern English, Black Country English, The Queens English and also I'm not sure about Newcastle-upon-Tyne if thats related or a completely separate language.