The show was broadcast 45 minutes earlier than normal, with horrible lead-in and lead-out shows (check the half-hourly ratings), on a bright sunny day of a four-day holiday weekend, and almost no advertising.
And in spite of all that, when you figure in the number of people who have simply stopped watching TV altogether, the season opener pulled barely one percent fewer of the ratings share than last season's. Which, incidentally, is over ten percent of the UK's entire population. The show is doing GREAT.
Now, this might actually matter on an American network, where the only thing anyone gives a shit about is Nielsen ad ratings (I'm becoming increasingly surprised that the USA hasn't started selling ad space on its fucking national flag), but the BBC is funded with license fees, so Doctor Who is not going away anytime soon.