Normally the holiday entitlement should be calculated on the terms of the contract of employment and/or the company handbook.
Failing that, the default position would be based on statutory minimum of the equivalent of 5.6 weeks per holiday year. For irregular hours it would usually be calculated using the accrual based on 12.07% of the hours worked.
I would strongly suggest having a read of the ACAS guidance on holiday entitlement to make sure you are compliant.
When it comes to paying the holiday pay, please bear in mind the calculation method from ACAS as it it a bit more complicated https://beta.acas.org.uk/calculating-holiday-pay