"...This calculator can be used to estimate the amount of Income Tax (not other taxes such as Capital Gains Tax) and National Insurance contributions you can expect to pay on your income – and find out how this money gets spent by the Government. It is aimed at people who pay tax through Pay As You Earn (the system used by employers and pension providers to deduct tax from your wages or pension). It's free and easy to use..."
So, according to your situation you can choose what calcultation is (more) suitable.
Last year the Treasury announced that in 2014-15 each taxpayer would be sent a document showing exactly how much tax and national insurance they paid and how it had been spent.
Because all in taken directly from their wages over half of taxpayers do not know how much income tax and national insurance they pay in a year. As in France, everyone has to declare in February what they have earned the previous year and according to the amount, there is a type of bracket in which you need to refer to and consequently pay that percentage. It is called "Déclaration d'impôts". This fiscal duty is compulsory for all.
If not completed in time: a fine is applicable.