I think she is married to an Australian and had been living there recently, maybe she picked up a bit of an Aussie accent mixed with posh English, maybe easier for her to pick up different accents as I imagine she has been all over the world during her career or maybe she just likes this one.
I met Darcey Bussell a few years ago. She certaintly didn`t have a Mid Atlantic accent when I spoke to her. She had a mild Australian accent which makes sense as she is half Aussie.