It's a sex discrimination thing. During OML and for babies due from this month, during AML as well, you are entitled to all your contractual benefits except pay. This includes paid leave. So the fact that your employer normally specifies that 8 of those days must be taken on specific named days doesn't mean you should miss out on them.
The OP's contract makes it clear that she is entitled to 8 extra days leave on top of her annual leave entitlement, which would normally be taken on the actual bank holidays but may involve a different day taken in lieu instead.
NL it was unclear for a while but I believe there was case law exploring it, can't remember when though, brain's fuzzy at the moment.
Adding to that, with the statutory holiday requirements increasing, for many employers the 24 days includes at least some bank holidays, and when it goes up to 28 may include all of them anyway, so there will be no ambiguity.
IIRC a few years ago when I was last in a 'proper' job and this issue was unclear, we did not make clear an entitlement to paid bank holidays in addition to the annual leave entitlements in contracts. I think we used to say something like the company may choose to observe bank holidays and the office may be closed, or some such nonsense. But basically they always did and everyone had a paid day off so that wouldn't have helped us really.
Bank holidays aren't any different in law from any other holiday really, the only difference is that they may be in the contract separately and the employer may specify when you take them, although with people working them as the OP can, they might be taken at any time in any case.