Here is a good blog I follow and analyses just about every aspect of the coverage in an impartial sort of way:
http://f1broadcasting.wordpress.com/...ease-slightly/
It seems both channels had an increase in the first half of last year but the second half suffered possibly due to the boring season. The BBC remained popular though and basically made sure F1 was even worth a mention in the UK. Sky lost 12% of their viewers overall in 2013 when compared to the year before which is a hell of a lot in the grand scheme of things. A few of my F1 mates have cancelled their subscriptions since the new year due to increasing utility prices and a lesser need for pay TV.
F1 is declining unfortunately at the moment and I can't see it improving. The BBC are losing interest and winding down the operation it feels, and if Sky get sole coverage, we can more of less kiss goodbye to F1 maintaining its 20 year high of popularity.
Its frustrating to watch because the BBC are holding the mantle of primary F1 coverage yet appear to be less convinced about holding onto the rights, yet Sky appear to be making an effort and attracting an embarrassing amount of viewers. Oh well, I'm sure somebody near the top knows what they are doing whilst the rest of us start to lose interest lol.