What's a negligible effect in an election?
One possible rule-of-thumb for researchers
One of the quantitative methodology papers which has most shaped my thinking in recent years is Carlisle Rainey's Arguing for a Negligible Effect.
You're using two different measures of "skew": endorsement (print media), and imbalance in coverage (BBC), and the headline conclusion depends on the content analysis of the BBC's coverage, omitting any consideration of ITV or Sky.
It seems to me that it would be better if you were able to rely on a single measure…