Hi
If you are entitled to receive a personal allowance, this would be deducted first.
Of the remainder, the lowest rate of tax would be applied first (20%), followed by the higher rate (40%) and lastly the additional rate (45%). depending on the amount of taxable lump sum.
More information in icome tax rates and personal allowances can be found at:
Income Tax rates and Personal Allowances
Thankyou.
Regards.