He sounds consistently English on his audiobook, "As You Wish."

Unlike some other Robin Hoods, though, only he can speak with an English accent.
James Oglethorpe: Philanthropist by day, Philanderer by night

He was born and raised in the UK but moved to the US for college. And lived in the US mostly since then. That would explain the half and half accent which is probably how he normally speaks. But as seen in his movies, both his American and British accents are flawless!

I'd put it down to the fact that he's done so many films that required an American accent. I love his old British accent but now he has this interesting mix of British and American that I love.
BOOSH BOOSH STRONGER THAN A MOOSE
DON'T LOCK YOUR DOOR OR WE'LL COME THROUGH YOUR ROOFTOP