Native speakers do not agree on what Standard English is. Almost everyone believes that what they speak is standard and other people have dialects. Many writers describe it in dictionaries, grammar books and style guides. Some of these publications tell the reader what the writers think English should be. Others describe how English actually is.
Some countries have standard languages; these include China, France, Germany, Norway.... The government can change words, grammar, characters and spelling in those languages. English has no such official standard and none would be tolerated.
The question is what kind of English should be learnt and taught? Written English is far more standardized than spoken English. So it is natural for second-language learners to focus on it.
Many local areas, in the UK and around the world, have their own words. For example, shroff, dim sum and congee are Hong Kong English and not UK nor US English. To ask for a cup of rosy is a local word for tea, found in only part of the UK. The word hydro means electricity in Canada, but in the rest of the world it is a prefix meaning associated with water.
There is a difference between educated English and common use. Words like: gonna, what's up and hang out, (going to, what is happening, socialise) are in common use. But these words are not accepted in books published for educated people. This type of difference is often a matter of class. The middle class tends to be the best educated and slang is not often socially acceptable in middle class society.
The function of language is to communicate. The best standard is to look for what is often called the common core. Those English functions that are used in all varieties around the world are the common core. When there are two spellings for words like centre/center, it is easy to choose one form to fall back on. Some choice has to be made, it might as well be the most common UK spelling. However do not fall into the trap of preferring anything that seems more British. The word bobby means a police officer in the UK, but not in most of the English-speaking world. Whatever form of English is taught it must be clear and usable worldwide.