"We have always been able, on the authority of the home secretary, to sign a warrant and intercept a phone call, a mobile phone call or other media communications[...]
This argument always makes me a bit angry, because it's a real example of the slippery slope in action.
Yes, law enforcement could always get warrants to obtain information that exists as a byproduct of providing services. But that's an entirely different thing than requiring people to damage security or gather extra data just so the government can have access.
In the US, CALEA was the atrocity that blurred these two things and indicated just how perverse the attitude of the government has become. Cameron is taking this notion to next level.
There have been a small number of studies that have shown a slight effect. However, to my knowledge, they have all been pretty flawed. And even if (for the sake of argument) they weren't flawed, the effect shown was trivially small.
It has been known for at least my lifetime that Nielsen ratings are rather questionable. They keep being used because there isn't a source of better data. Also, at least in theory, the errors in Nielsen data happen equally across the board. So when they say X number of people watched a show, that's almost certainly an overcount. But when they say X% of TV viewers watched a show, that's likely to be pretty close to accurate.
Advertisers know all of this, and the rates they'll pay take it into account.