When I was in the musical Scrooge, a friend of a cast mate came to see the show. I did not get a chance to meet said person, but the cast mate informed me that her friend believed wholeheartedly that I was in fact, British. (Cockney, to be more precise. I know that true Cockneys would want to make sure I pointed that out.)
To be perfectly fair, this person was not British herself, and may have had very little exposure to actual Cockneys. That notwithstanding, my accent was something I worked very hard on for that show. The outcome was pleasing to me. Being mistaken for a British person by an audience member was still high praise to me.
My method to bring the proper Cockney accent to life started early on in the production. My approach started out quite scientifically. It involved going to websites and reading linguistics analysis articles. Such delightful readings included pronunciation key spellings of common words. Charming expressions such as "asphyxiated glottal grunt" were used to learn this dumb Yankee how to speak all British-like.
About two weeks into that adventure, I packed it in, and adopted a different method. I watched a few hours of British sit-comes on public television in the evenings before rehearsal.
Scoff if you will. Be it known, however, that several of the characters in said programs had just the perfect accent I wanted for my character. So I set out to listen, very intently not only to what they said, but how they said it. I would set out, at first to imitate an repeat the actors on the show. When I was satisfied with that, I tweaked the voice I was using to my own satisfaction, and made it unique unto myself. Throw in the lines written in the Scrooge script, and presto. English guy. A reasonably convincing English guy at that.
I am sure the anthropological and geographical studies undertaken by the linguistics papers that I read are of extreme value. Yet for me, an accent and a language is more than where one places the tongue. The nuances of a Cockney accent (or any accent) transcend the shape of one's lips when one speaks. Language is a story in and of its self. To listen to the characters on those programs presented the accent in its natural habitat. Emotions, spirit, and humanity were all behind the accents there on the television shows, and hence presented a far more useful, not to mention natural exposure to the way one is to speak with an accent.
Accents and languages have poetry, and rhythm and humanity. Three things, I have to say, were totally lacking in my explorations into the "asphyxiated glottal grunt."
Being scholarly about it has it's place. For my money though, nothing beats listening. Just, listening.