some expenses are allowable in other counrties and not in the UK. Its like if you worked for an airline in Belgium you would get a more generous per diem than in the UK. The per diem is not part of your taxable income, when you send in your taxable income to the UK tax office it is calculated on your taxable income.
In many cases your total income and your taxable income might not be the same which can lead to tax savings.