Adele has a British accent as she was born in Tottenham, which is in North London. One of her crowning achievement is winning the Brit Awards for British Female Solo Artist and British Album of the Year.
Her accent comes from laziness, Ignorance and fashion. Her accent is not British, that doesn't exist as there are far too many regional accents. The BBC received pronunciation accent might be considered definitively British by some and elitist by others. One thing is for sure: the horrible North London noise that Adele makes hurts the ears of anyone who cares about language. It's fascinating she drops this vile massacre of vowels when she sings.