Re: Readers in North America should be aware of the differing UK terminology
That was made pretty clear by the article. It's also actually a more reasonable definition of middle class. I think the version that America uses is "pants". It obscures the difference between wage earning and wealth generation (and probably intentionally so). You shouldn't be called middle class unless you own income generating property. Middle class originally meant merchants and such, not affluent workers.
If you have no income generating property, you're just "working class" regardless of how large your debts are.
As a plumber, you should only be considered "middle class" if you own your own shop. Otherwise, you're just another employee. Tradesmen who are business owners can make mad money exploiting the labor of others.
Regardless, the definitions should not be engineered to include certain percentages of the population.