You will not be asked to apply the theories studied in metaethics to the issues studied at AS.
Metaethics asks questions about ethics. When you study medical ethics, you ask whether abortion is right, whether euthanasia should be legalised etc. You may use a number of different ethical theories to answer these questions. When you study metaethics, you ask about the status of ethical rules. Metaethics is not applied ethics - you shouldn't ask 'According to an emotivist, is it right to experiment on embryos?' The sort of question you might ask would be, "According to an emotivist, what do we mean when we say 'It is wrong to experiment on embryos?'" In other words, metaethics is concerned with the first part, 'It is wrong...' and is not concerned with medical ethics at all.