If you happen to find yourself in a working class pub near the east side of London and find you can barely understand what anyone is saying, don’t panic – your English is okay.
The reason you’re not understanding much is because the locals there are likely speaking with the ‚Cockney‘ dialect.
So if you’re planning a visit near London, watching the above video will help you.
You may wish to learn some of the top 100 Cockney rhyming slang as well: https://londontopia.net/londonism/fun-london/
Good luck and have fun!