have they changed your shift permanently, or have they just said that on weeks with a bank holiday you must make up the time on a different day.
I am not 100% certain, but i do not believe any employee has a legal right to bank holidays, and an employer can request employees work them, or make the time up. In my company, bank holidays are pro rata'd for part time employees,and once they reach the pro rata amount, they either make the time up on another day or use 1 day from their annual holiday entitlement.
Do you have a contract of employment which specifies your hours of employment.? I think that even if you do, employers are allowed to change your hours as long as you are given notice in writing, and i think there may be a legal minimum notice period, but i do not know what it is.