OP, it sounds like your company are actually paying you your holiday days on the bank holidays you work as opposed to double pay, which would then equate to 20 days annual leave you can choose and the extra 8 that you company’s says you have to work (bank holidays).
Worth noting that your company can tell you when you have to take you holidays and when you cannot.....
https://www.gov.uk/holiday-entitlement-rights/booking-time-off-
When leave can and can’t be taken
Employers can:
- tell their staff to take leave, for example bank holidays or Christmas
- restrict when leave can be taken, for example at certain busy periods
There may be rules about this in the employment contract or it may be what normally happens in the workplace. The notice period for this is at least twice as long as the leave they want their staff to take.