There's no denying that London's an expensive city but there are plenty of options available for those on a budget: http://www.gumtree.com/flatshare-offered/london
And all of those rentals will require at least a month's rent up front, which is the main point that the article was addressing - starting out renting is beyond the pocket of many people.
I wouldn't enjoy living there, but it seems better than the boat.
Gumtree displays 55 ads per page. So, after looking through ~0.25% of the adverts, you concluded that there was nothing cheaper than £300/month? Wow, that's pretty thorough!
It took me less than five minutes to find a hostel-style roomshare for £7 per night (less than £220 per month).