And rents are enough to pay for maintenance, which you have to pay for whether you rent or not. Just try re-selling an uninhabitable investment home.
And you need to be a more critical reader. The guardian article is full of red flags. It "claims" 57k vacant homes in London, where there are 8.5M households. So less than 7%. How many do you think should be vacant, considering that homes are vacant when for sale, when waiting to find a renter, and when waiting for the owners to move back if they worked out of country for a few years?
Secondly the paragraph on "buy-to-leave" investing indicates that rental regulations are so absurdly onerous that it is keeping properties off the market. Reforming them is the real solution.