Sorry to say it OP but they aren’t doing anything wrong.
During my old career in nurseries, we all got the minimum annual leave entitlement and bank holidays were included in that.
So for full timers like myself, 28 days a year. So, 8 days of that were the bank holidays. Of the 20 days remaining, 3 got swallowed up in Christmas closure week (so again no choice but to be off that week) so it worked out as 17 days off to put where we wanted. 3 whole working weeks plus 2 days. Yeah it really wasn’t much and must of use went through the first 6 or 7 months of the year before starting to use it for a summer holiday.
If you were part time, it would depend on which days you did. So if you worked Monday Tuesday Wednesday, you would have to remove all bank holidays from the year which fell on those days, but not others (eg Good Friday). But your total annual leave days would be less anyway because you work less days a week, so it evened out, maybe you gained an extra day due to less BH days but it wasn’t anything to be too thrilled about.
Draining but not illegal. Some companies are nicer and give you the bank holidays anyway don’t count them as your time. Some increase holiday leave with length of service. It sucks but sadly your company aren’t doing anything wrong.
If it’s not going to work I would just start to look round. And sadly asking to change day on bank holiday weeks to gain more time, you won’t be the first and it will be met with eye rolls and likely turned down, because at the end of the day everyone will want that.