It is odd that "NI contributions" should be referred to as anything but one part of income tax, the money just goes into the general chest - income taxation starts at the lower NI trigger limit, not the other.
Personally, I think all this talk of annual or monthly salaries is irrelevant while it is ascribed to the salaried/wage-earning working individual - as it is the sums being quoted are the joint income of the individuals plus the government. Why cannot incomes stated in employment contracts (full/part-time, long or short term) be stated nett of all state deductions/confiscations - i.e. as the individual's actual, genuine earnings for his/herself ? After all, the state has already made employers responsible for the collection of all taxes, fees, etc. and the employee never has anything to do with its calculation, far less seeing the gross income to (happily or reluctantly) hand approaching half of it over. The result would be clarity which I cannot see anyone objecting to (except those who love opaque systems, and there are quite a few of them). Would this really be so difficult ?
Self employed individuals are already treated as businesses responsible for reconciling their own affairs.