Hmmm. You haven't had the meetings which form part of your apprentice contract and your manager has refused to meet you to discuss your rate of pay, which you believe is below the national minimum wage. That's really not good.
If I were you, I would make an appointment with Citizens Advice to ask about the exact wording of your contract and whether the training you are receiving might be held to make up for the lack of minimum wage. It takes time to get a meeting. If your boss meets you in the meantime and you are satisfied with her responses, you can cancel. If not, go to the meeting, and ask their advice.
If they suggest that you should escalate, you could go to an industrial tribunal. This is a big thing but they provide good support to employees. You might need a solicitor, but there are briefs who will take on industrial tribunal cases under Legal Aid or no win no fee.