The Christmas airing of Doctor Who averaged 10.3 million on BBC in the United Kingdom. Although it was broadcast on the same day here in the U.S. on BBC America, I haven't seen the ratings yet for Saturday (Christmas day). I'll post them as soon as I can.
It was the sort of episode where I kind of wished BBC America had cut a deal to have it air on Syfy or another network with a little bit of promotion as a way of drawing in more viewers. It was an extremely accessible episode that could be enjoyed even by those who'd never seen an episode of the show before.
Update: the BBC America airing averaged 727,00o viewers from 9-10:20p on Christmas night.