Seeing that I want to get into Cambridge Engineering, I would need to make a competitive offer (hence the Further Maths). So, yes I am certain that five subjects (or 6 including Maths) is what I want or need to do.
Also I intend to go into Biomedical Engineering which is why I need the three sciences. And I only want or need to do CIE Maths because I want to do FM in year 13. Otherwise I wouldn't have done Maths and would have only done four.