Employees are entitled to a minimum of 5.6 weeks paid annual leave. This is 28 days for someone who works five days a week. Bank and public holidays may be included in the minimum entitlement and there is no statutory right to be paid bank and public holidays on top of that entitlement. As a part-time worker you are entitled to the same amount of holiday pro rata. I understand that you work Monday and Tuesdays each week. If these two days are regarded as full days you would be entitled to a minimum of 11.2 days holiday each year which would be rounded down to 11 days. It seems from your email that you are receiving holiday entitlement in excess of the minimum entitlement especially since it is possible that six hours each day (totalling 12) would arguably not be a full working day.
Your employer has in the past given you a bank holiday off for free, by which I assume that you mean that you have been paid for that bank holiday without it counting towards your holiday entitlement. Your employer appears to be stating that your holiday entitlement has been reduced to three weeks (presumably pro rata) as a result of other workers, who do not get the benefit of the free bank holiday, complaining.
Your employer cannot suddenly reduce your holiday leave entitlement without your consent and without consultation. If he does so, then you potentially have a breach of contract claim which could entitle you to resign and make that claim. Also you might have an unlawful deduction of wages claim if the likely result of this is a change/reduction in pay. From a practical perspective, however, it is unlikely that you would want to resign over this issue. Instead I would suggest raising this initially informally indicating that as far as you were concerned this was part of your contractual terms and it is not possible therefore for your employer to make a unilateral change just because others had been complaining. If that does not work then you could raise a grievance about it. Failing that, you could resign, but I would have said that it might be better to try to negotiate either a slight increase in pay or some other concession in kind rather than taking such a drastic step.