It's actually possible for me, a not-especially-good junior/mid developer to consider living alone with a <40 minute commute in London; that seemed to be a no go for guys making far more than me in SF.
All that being said, I saved a stupid amount of money in SF working on intern level wages. If your goal is to slum it and accumulate money, London does not come close. SF is a place to go and pile up money with a get out plan in mind.
Talking to people from around the US, it's just generally cheaper in other places in the US. Gas is cheaper, insurance, (non-SF)rent etc.
As far as space is concerned, in my experience one of the Bay Area's biggest problems was inefficient use of space in every which way possible. Even beyond the lack of usage of vertical space, every room I had was pretty huge, double bed, desk, room to do just about any kind of yoga I could think of and still a ton of extra on top of that. I'd have happily traded half of it for a few hundred off my rent.
I'd need to know what metrics are being used for quality (e.g. I'd take an absolute slum of a single room over sharing in prime apartment).
For consumer goods it seems they're comparing a cheap UK supermarket to Whole Foods in SF, and for rents they must be ignoring square footage and/or using different definitions of city center to get those comparisons. I checked out rental prices in SF on a recent visit and found SF pretty good value compared to London.