witchway,
That might be fairer, if people's outgoings scaled proportionately too. Wealthy people do spend more on food, housing, utilities etc, but it doesn't come close to increasing at the same rate their income does. Person A on £1,000,000 a year doesn't spend 100 times more on food or electricity or whatever than Person B on £10,000, therefore Person A has a much higher proportion of their income as disposable... Person B more than likely has no disposable income at all.
25% of each of their incomes (while theoretically the same proportion) has much more significance for B than for A... £250,000 sounds like a lot (is a lot!), but leaves A with £750,000 to cover their essentials. Even in a £5million mansion, they'll spend less than half that on a mortgage, so they'll have to struggle by on a mere £400,000.
Person B is left with £7,500... call it £600 a month. Say B is lucky and/or frugal and pays £400 mortgage (two thirds of their income), £50 utilities and £50 food... That £200 tax she pays is twice as much as she has available to spend on everything else in a month.
So... no. Although it looks reasonable on the surface, a flat tax rate is desperately, gaspingly unfair. It completely lets the wealthy off their obligations and amounts to little more than vindictive punishment of the poor. 25% is an almost insignificant amount for the very wealthy and could literally be a life or death sum for the very poor.