Ok.
A full time employee receives all 8 Bank Holidays as paid days off. A person working 2 days per week (40% of a working week) receives a proportiinate amount. 40%.
So as an employee working 2 days per week, in addition to your annual leave, you will get 3 Bank Holidays.
Say (for the sake of argument), your workplace is always closed on Bank Holidays and you get 12 days annual leave (swap in your amount here).
So..
You work 2 days per week
You get 12 days annual leave.
You get 3 Bank Holidays.
If you work every Monday, for every one of the Bank Holidays your workplace is closed you either need to "take a Bank Holiday" or "take an annual leave".
There are 8 Bank Holidays and you get 3. So to cover the remaining 5, you need to use 5 annual leave days.
This leaves you with 7 annual leave days to use when you like.
however
If you work Tuesday and Wednesday, you don't need to cover any Bank Holidays (unless the christmas/NYD ones fall on these days).
So then you would have the 12 annual leave days plus the 3 Bank Holidays. So you would get 15 days off to choose as you like.