You must use the total holiday allowance including bank holidays to calculate the pro rata holidays for a part time worker. E.G. you must use  28 days and NOT 20 days + Bank Holidays.   If you pro rate the  FTE bookable days e.g. 20 days  + bank holidays that fall on work days this is potentially unfair and unlawful.

It is potentially unfair  as an employee working part time, say, 4 days of a 5 day week,  Monday to Thursday would get 22 days leave, i.e. 16 days pro rata leave, using 2021 as an example would get  + 6  bank holidays that fall on work days. An employee who works Tuesday to Friday again using 2021 as an example would only get 19 days leave, i.e. 16 days + 3 bank holidays that fall on work days.

It is potentially unlawful as the employee who doesn’t work Monday would accrue leave at 10.05% per day compared to a full time employee who would accrue leave at 12.07%  per day, in breach of the Part Time Workers no Less Favourable Treatment Regulations.