Tax is paid from the first pound earnt, and offset against the income to create an effective tax free allowance. So for example, a £10k citizens income combined with a 33% tax rate creates an effective tax free threshold of £30k. If you withdraw pound for pound, you risk creating a benefits trap as attending work has a cost, both financial and time, so you need to mitigate that.
A slight variance on the NIT then, but explain, since I guess this would also be getting rid of the minimum wage, if the citizens income is for example 10k (still under a living income), and basic menial jobs pay much less, then isn't this leading to just as much disincentive to work as the benefit traps we have now?