'Progressive' as used in the same context as your original post is basically a placeholder for 'benefiting the groups I care about'. It's not a meaningful word to use, and you even know that. Contrast with 'progressive tax', which
is well-defined.
I don't understand the line of thinking which effectively says "people don't want to earn £2 if they have to pay £1 in tax". Erm, you're still richer than you were if you didn't earn more. The only possible argument against this is if earning another £100k takes more amount of effort it took to earn your first £100k. But if you know anything about making money it doesn't work that way. With more capital comes greater opportunities. You can afford an accountant to manage your money, you have more investment options, and so forth; and you still have enough to spare for a comfortable lifestyle.
A flat tax implies that a dollar is worth just as much to Warren Buffett as it is to someone flipping burgers, which demonstrates a complete lack of perspective.
Put it this way: everyone's living expenses are, say, £12k. now if you're earning a median wage that leaves you £10k to invest how you like. If you're earning £50k you get to invest £38k. assuming the schemes offer the same rate of return, guess who is going to amass £50k first? Hint: it's not the guy who has to get a 5x return on his investment.
The higher rate income tax is an incentive to invest the money you receive back into the economy via business or charity, where it is taxed at a lower rate or even not at all, rather than spent on your own personal gain. Why should it scare you that the government would take that money? BTW, investment in financial instruments of dubious benefit is giving money to the financial system (this should really be obvious...) so it does contribute to the economy, but there are almost certainly better ways to do so.
The problem with the libertarian perspective is that it irrationally assumes the best of people when there is no incentive for them to do so. If a billionnaire can afford to fund all the services he requires that someone on the poverty line would depend on government for, why should the billionnaire give his money away? If you want a society you have an obligation to provide for one, if you believe it is unfair that someone with broader shoulders should carry more weight, then perhaps you should consider how fair it is that he has broader shoulders and others do not.