The Sun has consistently been the biggest daily in the UK for a long time, and I would suggest at least some of the parties have a majority buying that paper, historically the Mirror which comes in 3rd would be considered a more leftist paper (as was the guardian itself) and so would be much more likely to be a favorite amongst Labour voters.
The Mail hasn't had daily sales over 4 million this millennium. But since you apparently overestimate the guardians sales by 100% too I guess the point you're making still stands.
http://en.wikipedia.org/wiki/List_of_newspapers_in_the_Unite...
apologies - I was given these numbers while training as a political party's election agent - I didn't know they were that stale!
Thinking about it, they may be talking about readership rather than sales. People can pass around a paper in the break room or family members may read the same paper. I'm not sure how well they can estimate those numbers though.
Sales figures obviously give a much more concrete number for comparing paper to paper, but if you're trying to make a comparison to TV viewers or some other type of media maybe the readership numbers are more meaningful.
Actually, a lot fewer newspapers are sold in the UK than that, according to this: http://en.wikipedia.org/wiki/List_of_newspapers_in_the_Unite...
It says the Daily Mail has a circulation of about 1.8m daily, the Guardian has a circulation of about 200k daily. The Sun is the most widely bought newspaper, at 2.4m daily, but they're all declining across the board.
(aside: the majority of voters for all three of the big parties in the UK read the Daily Mail, of them that read a print newspaper.)