Can you back up the claimed existance of that bias by actual scientific, methodological sound, research? And not by 'Should be clear to anyone!11' or 'just read this article I don't like'. I know of only one study, in Belgium, that showed no left-wing bias and a very slight anti-sitting government bias.
I don't need one for The Guardian - they're quite open about their bias
https://www.theguardian.com/gnm-archive/2002/jun/06/1 and considered themselves the "voice of the left" for several decades.
As for the BBC, the outlet has faced almost non-stop controversy over its biased reporting, to the point where they even caught the attention of government regulators ~
http://www.telegraph.co.uk/news/2017/03/22/stop-bias-against-brexit-face-fine-bbc-warned/Not that I need even post any of those links. All the empirical required to infer bias could be collected by oneself, given that we define media bias as "the bias or perceived bias of journalists and news producers within the mass media in the selection of events and stories that are reported and how they are covered." The disproportionate coverage of topics and events conferring a postmodernist or progressive message against those of a conservative nature, proven by the quantitative amount of articles produced by each platform's outlet, would be sufficient to infer bias within itself. Not to professional standard admittedly, but to a sufficient degree for a dying internet forum. I'd conduct it myself but it's not my definition of a good Saturday evening.
Would you rather I post an amateur study group for you to compare their methodology against this week's PoliSci lecture notes?