Paul is right. 
Please allow me to say one thing as a non-native speaker. 
We are not stupid people who cannot feel the differences between colloquial things and formal things. 
Non-English speakers know other languages. In most languages, there are differences between "informal" and "formal". Russian, Japanese, German, Chinese...

So we do know when to use and not. And if we say something wrong in formal situation, you can just say that we shouldn't use it and correct us in a nice way. 
And why can't we use those expressions? But others use it. What if your boyfriend/ girlfriend uses it, but you cannot use the same word to others? I really don't like it when people say "You shouldn't use this colloquial expression because you are a foreigner." Why not? It's not like we are swearing to you.
It happens in Japanese as well, some old conservative people say "oh no, that's rude if you say it, because you are not Japanese." I think it's kind of racism.  

Awesome article!! I've lived in England have been speaking English for a long time now.
I also think that that's how people speak. And as a non-native, I know that a lot of non-native speakers wonder what they mean when they say "And I go~". We would think that it literally means "GO".
So this article will help a lot of students and it is almost like a secret. Because teachers or native speakers don't tell you. They use these without realizing and say that it's not "sophisticated", but I've heard so many people using this. This is the real English, I would say.
Even for other languages, for example Japanese, we have very strict formal language, but no one actually speaks like that in real life. You would end up sounding like a robbot or textbook. So it is important to learn colloquial things. I honestly don't understand why some people just deny the fact that people DO speak like this. Did Ben tell that you MUST use this? No, it's your choice. But for non-native, it's good to know what those colloquial expressions me.  
Languages change. Shakspeare created loads of new words. Non of his new words made any sense first, but now we use all those words he invented.
And it's just wrong and rude to say "WTF" or diss people who contribute their time and write these  articles for learners FOR FREE. 

'@Ben Its not correct to say only the less educated speak in this way, you are correct on this point. I too often speak and use terminology in the way that you mentioned, and like you I also studied at Oxford. But there is a key difference here between me and you and people who are learning English as a second language.

As native speakers we understand that there are certain occasions when it is not appropriate to speak like this and use slang in the way that you describe. We understand the nuances of the language, and the fact that the manner in which you talk affects the way that you are perceived by other people. We also know it is not correct to write in the manner that you described, except under specific circumstances.

Someone learning English as a second language does not have that knowledge, at least until they have developed an advanced understanding of the language.  Personally, I think the article should be a lot more explicit that

1) This is British English

and

2) That this advice is about the colloquial spoken language used in conversation only (not written English) and also that there are times where it is inappropriate to speak like this.

Taken with this prior knowledge, I can see how this article could be really useful to a learner.

Finally my advice to non-native speakers reading this article would be to note that sometimes native speakers of British English speak in this manner, and be aware of what it means but I would not try and imitate it unless you are at an advanced level in the language, and have the ability to recognise that the way you speak can be different for different situations, and can modify your language accordingly. In my opinion it is better learn a standardised version of the language first, and THEN adapt your use of language to the situation if it demands it.

People do interchange 'went' for 'said' and 'go' for 'says', that is quite common in the UK in spoken English.

But overuse of 'like' and 'ain't' in conversation are bad habits that native speakers have, but often use anyway in informal, spoken English. If you spoke like this in certain situations you would come across as either lazy or ignorant, or both. 

I don't entirely agree with the indirectness thing you mentioned. We do this, but certainly not for something as insignificant as asking if someone wants a drink. Here it is perfectly normal to be direct. This indirectness you refer to comes when people have to ask awkward or delicate questions, so they preface what they are going to say with words to make the question seem less threatening 'e.g. 'I'm really sorry about this, and I don't mean to be rude, but could you... etc'.

I think there is value in teaching students how to decipher the seemingly inconsistent use of the words "went" and "like" in the examples given but I would hesitate to suggest that students emulate these colloquialisms. So I find it useful to know these things in order to understand what you are hearing but I stop short at recommending them.