That is indeed possible. Basically, you get a minimum holiday allowance, under law or contract, whichever is greater. In your case you get a total of 30 days. The distinction between normal days and bank holidays is not that important in law, because they all get treated the same. The important thing is how many days’ holiday in total you get a year, which is the amount they need to allow you to take and get paid for.
If on bank holidays they do not need you to work, they do not have to give them to you off as holidays and can indeed ask you to make up the hours elsewhere. The important point is that they cannot then take that day off your holiday allowance and must save it and allow you to take it another time.
Also, the issue is what you can do to challenge this, which in your case is not much. That is because the only way to actually take any legal action is to resign and claim constructive dismissal by arguing the employer has committed a serious breach of contract. However, the catch is that you need 2 years’ service to do this so in your case you are not actually protected. The only other claim you can make, which disregards ***** ***** to claim if your employer has taken away your day’s holiday for a bank holiday and then asked you to make up the time elsewhere. As long as they allow you to take that day another time, there is no breach.
I'm sorry if this is not necessarily the answer you were hoping for, however I do have a duty to be honest and explain the actual legal position. This does mean delivering bad news from time to time. I hope you understand and would be happy to provide any further clarification if needed. If you are still satisfied with the level of service I have provided regardless of the contents of the answer, I would be grateful if you could please take a second to leave a positive rating by selecting 3, 4 or 5 stars above. Thank you