I certainly agree that there are media bias's (leaders like Corbyn and Foot and even Kinnock before him didn't stand a chance).At the same time, however, it is important to take into account and not ignore factors things like media bias. If you look, for example, at major news outlets and which side of the debate they endorsed, the circulation for papers backing the leave campaign was far higher than the remain. The same can be said about how the media support different political parties and the impact that has. Look at the conservatives today and Labour with a certain Rupert Murdoch paper in '97.
In Brexit though I would say it was the high circulation (as you say) Sun, Mail and Express against all the rest of the media including the Broadcast media as well as the government at the time and most of the parties.
I don't therefore see an overall bias in the media to Brexit and perhaps even the opposite when the government propaganda is taken into account.
Obviously just my assessment of it but I've tried to be unbiased.