Hello lolobolo.

Sorry I could not see the link you posted so cannot comment on the note.
The UK is made up of many regions which all have regional variations to langauge. I imagine when taught as a language it's taught in it's pure form and many people experiencing English as spoken by the "natives" for the first time maybe supprised we do not all sound like the news presenters.

I have to say many accents I struggle to understand and I live here...

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We also have a wide ethnic mix and people for whom English is not their first language will bring something new and different to the mix. Of course there is no excuse for bad grammer etc.. everyone is taught this at school but I think some are very lazy putting it into use preferring the use of slang words. This I think is a great shame.
The UK is made up of four separate countries England, Scotland, Wales and Northern Ireland. I'm sure you're aware of this, so not only do you have the regional accents to cope with but in the case of Wales a completely different language of their own , Welsh or Cymraeg. Scotland and Ireland also speak a langauge ~ Gaellic, but it's not as widely spoken as welsh.
It's a shame you felt let down by the people you met and their language skills. I hope it did not stop you enjoying the rest of your trip and that you'll vist the UK again in the future.