I can indeed argue that, since I argue that the income ratio is improperly distorted by the expenditure imbalance.
To put it another way, one could simply apply a correcting factor to income to account for this distortion. Or one could simply compare for people living within their means.
The distortion factor renders the raw results somewhat moot: if a household with £10k disposable income spends £15k, then of course that household is likely to spend a much higher proportion of its "£10k income" on VAT.
Indeed, it would be statistically (and philosophically) correct to say that where poorer households are spending more than their nominal net income, one should in fact add the amount of debt to this income figure. (And that's without even considering* the fact that wealthier people save far more proportionately than poorer people, which again distorts the "nominal net income" ratio).
* Not that this should be considered in the comparison of course - I'm just articulating it as a factor.