In 1985 a black man in London asked me directions. He addressed me as “mate,” and I could barely understand his Cockney brogue but understood just enough to help him. And he apparently understood my California accent.
I was there for 3 years and so I could understand them quite well. Early on in my tour my wife and I would be watching a British comedy show and the audience would bust a gut laughing at a joke; my wife and I would look at each other and just shrug. However, we learned to understand them better and also learned some of the slang and so we came to think that British comedy is hilarious.
Also, driving on the left was never a problem for me.
Currently, 50 years later, we subscribe to BritBox which has a lot of British television shows.