Hello, Rai, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
Your commitment bonus would be part of your income for Income Tax (IT) purposes so one must assume that your P60 is correct.
Tax credits are calculated as shown here:
The following extract from this source is relevant:
'wages as an employee, including any ‘employer benefits’ you might have had'
Here is further support from Disability Rights UK web site:
'What counts as income for tax credit purposes is based on income tax rules. So taxable income is taken into account and other income is ignored'
Thus the inclusion of your bonus should be taken into account by the Tax Credit office (TCO) in assessing your entitlements for the tax year in which you received your bonus.
I am so sorry to have to rain on your parade.
Not very nice I know to receive a bonus and have your tax credits taken away. Always check the TCOs calculations though as that office is notorious for making errors.