From Outpost Gallifrey:
‘Unofficial figures show that episode three of Series Four, Planet of the Ood, was watched by 86.9 million viewers, giving it a 33.4% share of the total television audience.’
This makes Planet of the Ood the most-watched Doctor Who episode of all time, beating the previous champion, City of Death Episode 4, by more than 70 million viewers.
Yes, given the population of the UK is about 60 million they must be counting all the people >ahem< viewing it online!
Well, then they could be considering all the British people watching it again on their website, that would inflate the numbers.