Forgetting what the contract says for a second, how many holidays have you actually been permitted to take? - 20 pro-rata or 28 pro-rata (i.e. with or without the inclusion of BHs).
It sounds like you have previously been been granted 28 pro-rata, and suddently your employer has decided it wishes to change the terms of your contract. It can do that (though it ought to be consulting with the workforce first to explain the reasons why and discuss what issues it gives employees) but it is must notify you in advance, giving you at least your contractual notice period of the change.