To be honest, I difficulty stopping myself punching a hole in my computer screen while watching that video. The young woman's 'rant' said a lot more about her inflated sense of self-importance and her own skewed view of social hierarchy than it did about language.
The accent she was imitating at the beginning of the video is the stereotypical 'posh' plums-in- the mouth accent of the English ruling classes of yesteryear, with its unnaturally extended vowel sounds. Of course, hardly anybody really speaks like that. Even Prince William and Prince Harry don't speak like that! What's more, nobody is expecting foreign learners to imitate this accent.
As well as being intensely annoying, this young woman also fails to understand the basic difference between accent and pronunciation.
ACCENT indicates geographical region, and, to a lesser extent nowadays, social class and education. None of this is relevant to you as a non-native learner. Of course nobody is expecting you to imitate the accent of an 80-year-old English aristocrat! Nor is anybody expecting you to imitate the accent of a Manchester rap artist or a Glasgow bus-driver, either.
What you DO need to do, is learn clear and correct PRONUNCIATION. If you consult any good dictionary, you will be given two alternative pronunciations for all words - one is the standard American, and the other is standard British. The latter is RP. This is the way I speak, and it's also the way that most of my family, friends and colleagues speak. Call it modern RP, relaxed RP, call it what you like. It's not posh and it's not upper class. It's just neutral, plain, standard English, the same as in the dictionary. And this is the pronunciation which you should follow if you want to speak an 'ordinary' British English which anyone would understand.
Please don't waste any more of your time listening to that irritating young woman in the video.