Please note this well:
Gentzkow and Shapiro have studied this question in the US. Using an objective (if imperfect) measure of bias, they found that newspapers closely match the political biases of their potential readers, as measured by votes cast in the 2004 presidential election, and by the source of campaign contributions to each party. No doubt the causation runs both ways, but one striking result is that the proprietor’s identity seems to make no difference to the bias. The media barons tell us what we wish to hear.
Newspapers reflect our own prejudices back to us, not cause them.
The reason The Sun sells well is because millions of people like a bit of working class populism leavened with tits. The reason Socialist Worker does not sell well is because there are, contrary to general appearances among the population I admit, very few entirely insane idiots with cash money.