Often an employer will pay staff a higher wage for working on bank holidays, but this is not a legal requirement, and there is no formula saying how much more you should be paid for working any given hours.
Any extra payment is up to your employer to offer, and to state in your contract when you agree to take the job. However, if your employer has automatically offered it over several years (i.e. it has become part of your contract by 'custom and practice'), a refusal to pay the higher rate could amount to a breach of contract. Equally, if your contract states that you will get paid double on Christmas day and your boss is now trying to change that, this would be a variation of contract which can't be done without your agreement.
Consult your union rep if you have one, or seek individual legal advice if you think this might be the case. In a workplace where a union is recognised, enhanced payments and/or time off in lieu for shifts such as Christmas Day and other national holidays are likely to have been negotiated through collective bargaining and will be set out in the resulting collective agreement.