Ecclestone is making noises that F1 may leave free-to-air TV entirely in the UK, he also says that Sky gets 10m viewers while the BBC only attracts 6 or 7m. Where he gets his figures I don't know...
http://www.guardian.co.uk/sport/2012/jun/07/bernie-ecclestone-formula-one-free-bbc
http://www.guardian.co.uk/sport/2012/jun/07/bernie-ecclestone-formula-one-free-bbc
Not all of them watch the F1 coverage, although given the way Sky monitors your viewing habits (if you're stupid enough to connect it to a phone line) their viewing figures are probably more accurate than the BMRB stuff who take a small snapshot of viewers and then multiply it by the number of homes with a TV (I know it's more complicated than that).