OP, these figures are terrifying, but not for the reason you think. Firstly, they show that our economy is fucked. The income tax system is overly complex and utterly unfair. Not only that, but it's extremely precarious. The absolute mess the Government is making of Brexit means that many high-earners may soon decide to leave the Country, or if they don't, the companies that employ them will do so. It just so happens that, thanks for the dishonesty of successive governments, income tax only comprises a minority over the overall tax-take. However, we can't afford to lose even that.
I'm more troubled by the fact that over half the population pay no income tax. This is utterly unsustainable and iniquitous, and that's even before you even count total net contributions (tax paid in, minus services and benefits taken out).
I'm a high-earner and a lefty, and I'm one of those people you accuse of 'sneering' (are you sure that's what you mean?) that they'd pay more tax. I believe in a TRULY progressive tax system. That would mean the abolition of in-work benefits, child benefit etc, but the lifting of low earners out of the tax system. It would also mean closing the loopholes for those at the very top, who pay less tax proportionally than their cleaners do.
Maybe the only pragmatic answer is a flat tax, with a high starting threshold and the abolition of in-work benefits. The thing is, the Government would never implement this because it would be too transparent. It would all but eliminate tax 'avoidance' and make tax evasion extremely difficult and people would see how much they're really paying.
Our State already spend less as a proportion of GDP than almost every other developed nation (and that's including health; most others don't). Where do you suggest either the extra money, or the extra cuts in spending, come from?