Poshen

([…] He's a grammar-school lad on the make, who poshened up at Cambridge and now can't speak normal, which makes him sound ludicrous when he tries. Along the way he also poshened up his team, dumping Fulham to follow Chelsea.)

Most people tend to poshen up their accent if they go for a job interview and a lot of us have a Sunday-best voice for when we answer the phone or talk to the doctor. We save our real accent for our family and friends.