FWIW, The paper costs £1, so with a readership of 7 million or so every week... That's around £350m / year.
The top advertiser, Sky, spends £2m a year with the paper.
So I find it hard to believe that advertisers are all that important.
Minus the cut to the retailer, minus the cut to the distributor, minus the cost of printing the thing... The income from advertisers is a much higher percentage of the paper's income than you think.
Well, the Metro and Evening Standard are given away for free in London each weekday, and only make money from advertising.
The Metro is mostly recycled news stories and press releases from their sister paid-for newspaper, the Daily Mail; their costs are going to be a little less.
Tabloids traditionally make a higher percentage of income from the cover price compared to broadsheets, partly because their readers are less affluent and so less appealing to advertisers. Still, this 2002 article puts advertising at 40%, which sounds pretty important.