Hi
It is illegal for you to pay for holiday days/hours that haven't actually been taken (not sure if this is what you are saying or not but adding hol pay as a figure to hours worked and paying it seems like this).
Not sure if this is addressed to me, but to clarify: -
Our employees are employed on 'Zero Hours' T&C's.
This means that they are not scheduled to work any hours / days at all and thus booking a holiday is pointless as if they don't want to work they don't have to. To this end the law dictates that they still accrue holiday pay as they are not to be discriminated against because of the way in which they are employed.
We therefore make a payment twice per year, not for holidays not taken, but for holidays accrued.
Your statement would be correct in relation to part time or full time employees that were being paid for holidays whilst at the same time working them (i.e. holiday pay and hours worked for the same day).