Which accent does Carey Mulligan speak?

I wonder what kind of accent does actress Carey Mulligan in the Oscar buzz The Education speak?
Here's a video of her interview:
http://www.youtube.com/watch?v=qBWQGoiQals&feature=fvsr

In the film she has a traditional RP accent (Received Pronunciation). It's an English associated with the upper classes or educated people in England. In the interview - her genuine accent is less posh but still RP, and more like a general nondescript English accent, if there is such a thing!

thank you for your explanation. Is her natural accent called Estuary?

Yes, Estuary English is pretty accurate. If you asked me where she was from, I'd say probably the South East - but she could be from almost anywhere in England, and she might have modified her local accent to be closer to RP/Estuary English. She is an actor, and she has probably had vocal training of one sort or another.

RP has a kind of old fashioned sound to it in comparison to the modern Estuary variety. Even in the UK, RP can sound pretty stand-offish. It's not even as popular as it once was amongst the upper clases. Educated Estuary English is more or less the defacto English accent these days - heard on TV, radio, movies etc.