Donations under the payroll giving scheme are paid gross and are an allowable deduction in arriving at employment income. Therefore a smaller amount of employment income is included in the income tax computation which means that the correct amount of tax relief is automatically given. There is no further impact on the income tax computation.
Great answer! The same concept applies to the difference in treatment between personal pension contributions (dealt with like gift aid) and occupational pension contributions (dealt with like payroll deduction scheme)