Even though English is one language, there are different versions of it in a sense. Some English speaking countries have different words to mean different things from other English speaking countries. In some countries that speak English, they use a lot of slang words and phrases. Therefore this is not the purest form of English.
When someone says they want to speak the purest form of English, they’re referring to the way that English was spoken when it was first created. And most likely hood the Oxford English is probably the purest form of English. It is old and is most likely the first language of English before people started changing some of the words in it. The Oxford English has its own dictionary.