Current Model
 
The currently assessed income tax and NIC rates are shown in the table.

  -  There is an initial "Personal Allowance" of £12,570, which means that any income up to this level is not taxed, nor are NICs taken against it.
  -  The first rate is the Basic Rate, which is applied between £12,571 and £50,270, and is 20% for Income Tax and 8% for National Insurance, so 28% in total.
  -  Between £50,270 and £125,140 the Higher Rate of 40% is applied, plus 2% for NICs, so 42% in total.  However, between £100,001 and £125,140, you also lose 50p of your Personal Allowance for each additional £1 of income. Since each 50p that is taken off your Personal Allowance is taxed at 40%, you lose 20p for each £1, plus the 42% already being taken, so you lose 62p on each £1, or 62% in total.
  -  Last is the Additional Rate, for incomes of £125,141 and higher, where you pay 45%, plus 2% NICs, so 47% in total.

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Note that the rates paid are applied sequentially and cumulatively so that the total tax you pay will always be less than 47% (i.e. even if your income is $30m, you still only pay 28% on the first £38k, which is taxed at the Basic Rate).