If you are only getting the legal minimum of annual leave, it makes no difference what your usual working days are – your entitlement is based on the length of your normal working week, and you are entitled to the number of days you work per week x 5.6.
But if you work for an employer who gives more paid leave than the legal minimum, it is possible (though unfair) that you could lose out.
Entitlement to bank holidays and any extra leave above the 5.6 weeks' statutory minimum, is governed by your contract of employment. You should check your staff handbook, and discuss this with your employer.
Government guidance on the Regulations says: "Under the regulations, part-timers should not be treated less favourably than comparable full-timers in their entitlement to public/bank holidays. Allowing full-timers the day off, but not part-timers, is clearly less favourable treatment and unlawful under the regulations unless there is objective justification."
The guidance goes on to recommend that: "It may be necessary to remove the disadvantage suffered by those staff who do not receive particular days off as a result of their particular working pattern, for example by giving all workers a pro rata entitlement of days off in lieu according to the number of hours they work."
As this issue affects all part-time workers where you work, this is an obvious case to take a collective approach, to try to persuade your employer to change their mind, and to involve a trade union if you, and ideally other colleagues, are members.
If you are not yet a member of a trade union, browse our Union Finder tool to find the union most suited to your needs.