I agree with Eloethan that daphne's point is really important. Do we really think it is OK that the overall income of the very rich is taxed at a lower rate than the overall income of the middle-earners and the poor? The so called "squeezed middle" (earnings either side of £40,000) are, in addition to the squeeze on jobs at this level which is generally lowering salaries, increasing work-load or cutting jobs altogether, squeezed by the government/treasury too.
I also agree with NanBridget that transparency is the only way forward. Given that openness (of all incomes and tax) you would soon see the media adding the NI on to the tax of groups of people and, I am pretty sure, we would see that many very rich and rich would pay a lot smaller proportion of tax on their income (including "unearned").