There are many varieties of English and no one variety is linguistically superior to any other. We generally use a spoken language that is close to that of the people with whom we expect to spend most of our lives. For the British middle-class, that means adopting the dialect known as British Standard English. It can be spoken in any accent, but in the United Kingdom it is often associated with the accent of educated people living in London and the south-east of England. But British Standard English is no more and no less correct than Midlands, Liverpool, Tyneside, Indian, Australian, North American or Caribbean English.
Written language derives from speech. We learn to speak before we learn to write and for a long time there was no written language at all. There are still languages that are only, or at least predominantly, spoken. The written language is unlike speech in that we have to make a deliberate attempt to learn it. Some fail to do so, even when they speak their native language fluently. We have to encode our thoughts as arbitrary marks on paper or screen and interpret similar marks produced by others. Like speech, different kinds of written language suit different circumstances. An email or text message in a variety of language that many of us would not understand or appreciate is perfectly appropriate between people who do understand and appreciate such language. The question of whether or not it is correct doesn’t arise. However, such language in, say, a job application or a Times leader would be unacceptable, and consequently ineffective.
Those who commit words to print need to consider what they are trying to express, who their readers are, whether the chosen language will succeed in conveying the message clearly, and whether it is appropriate for the medium used. It is helpful if, in writing for a large number of people whose linguistic backgrounds we cannot know, we agree on certain conventions of punctuation, spelling, and choice of vocabulary and structures. In speech we generally know personally our audiences. In writing, too, we will sometimes know our readers and we can adapt our language accordingly. Quite often, we will not. In those cases, a certain commonality is required to avoid chaos.
When I read a sentence I ask myself not, ‘Is it correct?’ but, ‘Do I want to read any more of this stuff?’ The successful use of language means achieving an intended purpose, and that includes producing a sympathetic response in our readers. Placing the emphasis on effectiveness rather than correctness seems to me more likely to produce the desired result. The alternative is to suppose that once you have complied with the rules laid down by this or that authority you have done all you need to do. That is far from the truth.