as a direct response to some of your standings:
Firstly I'm going to be arguing from an egalitarian and individualist point of view, i believe that people should all have a basic ground work to start from such as good education, free healthcare and basic shelter and food upto a point.
Flat tax:
Seems ideal at first, Who could argue with a system where everyone pays equally and encourages people to seek higher wages?
However, you have to remember a few things, firstly there are very few jobs that pay the higher wages, thus one cannot simply work very hard and obtain a higher earning job, there are things such as luck, connections from family and starting wealth that may mean easier access to these kinds of jobs.
Now you can either take the stance that it's perfectly fine that they have those advantages and i would understand that view, but if you want to perhaps lower the influence of luck and work more towards a metiocratic earning system (a system where people earn based upon how good they are, ie on their merit) then you could levy a tax that you perceive to reduce the advantages these lucky people have, this could be in the form of inheritance tax, progressive tax, capital gains tax and property taxes. The first system by which there is a flat tax is more more free market driven than a progressively taxed society.
There is another argument where you look at the basic living standards, you could say for example that a person earning £150,000 a year earns more than enough to support themselves and their family, therefore they can "afford" to be taxed at a higher rate because their after tax income will still be high enough to more than comfortably support their family and set up future generations. You could also say that this would be like forcing charity contributions, imagine if the government said "you have to donate to this charity or jail time" while of course this analogy has holes in it such that charities are private while taxes are paid into a system voted for by the people, given power by the sovereign. Even then, however, you could say you are "oppressing" the minorities views by making them support a "charity" they don't like
That is the moral side of the argument, albeit a part of it, you could also argue that a flat tax doesn't bring in enough income to run things such as the NHS (which is great value for money) in which case we would use health insurance....so why not just have a progressive income tax?
There are two sides to all political ideas.