Firstly: Anyone who claims that the tax code is complex because of multiple tax bands clearly has absolutely no understanding of taxation. The UK tax code is something like 10,0000 pages long, with the separate tax bands taking up the whole of 2-3 pages. The US has something like a dozen tax bands, creating a much smoother function which has lots of benefits when going through pay rises, receiving bonuses , calculating deductions etc. It would be great if the UK moved to a more continuous function,; start tax at 15% and increment it by 2% each band to you hit the current 45%.
Secondly: you are on your own thinking there is a more issue with multiple tax bands. Countless philosophers and economists conclude otherwise, and the reasons are quite obvious. Money doesn't have a linear value for starters, and there is already a moral problem that salaries are vastly different despite people working similar hours and just as hard. Then there is the entire debate around the existence of taxation in he first place, but basically if you want to live and work in the state then the states' tax rules apply in absolute.
Flat rate doesn't replace the current benefits system, not without a significant UBI, but these are not necessarily linked. Current tax already treats everyone equally (all people earnings the same with the same deductions pays the same tax) and there is no moral case for avoiding taxes. The problem with flat tax is it creates a system that is extrmely sensitive to the tax threshold, and if implemented you will find a lot more people not paying any taxes what so ever, many more people paying at a higher tax band, and the wealthiest will carry on trying to avoid taxes. E.g. you set the tax free threshold to 20K and everyone above that pays the 45% tax rate. Someone that used to pay a little tax on their 18K income now doesn't pay a penny, their friend who earns 24K finds the selves with a very steep marginal tax rate, and they find them selves simply working less hours because anything earned above the 20K sees a large deduction from their paycheck. If instead you had 20 different tax bands then the change in marginal rate is so gradual it is invisible.
We've been through this. You think that the state is fine to treat people differently based on arbitrary reasons, as long as it applies those arbitrary reasons consistently.
I disagree, because the above flies in the face of equal treatment under the law.
The logic inconsistency in presenting your position as equality may help you sleep at night, but it doesn't make it true.
Seperate but equal is not a solution.