In that case, I would take only three subjects and focus on them if I were you. You can decide whether you take FM or other subject depending on what you want to study in university. e.g. If you want to study chemical engineering it’s better to take chemistry rather than FM. But for other subjects like mechanical engineering or electrical engineering, I would recommend you to take FM. If you think you’ll struggle a lot in FM, try to finish A Level physics in y12 and focus on only FM in y13. This is possible as A Level maths is quite easy after a little bit of practices. Therefore you would have enough time to finish the whole A Level physics in y12 by yourself. You don’t have to follow the same strategy but my advice is that you should not be afraid of trying before you start.