I agree that history is perhaps not the best choice given your current course plans. Chemistry is required for almost all medicine courses, and Biology for many. For those that Biology is not required, it is usually the case that the vast majority of successful applicants have studied it regardless.
Maths and Physics are vital for engineering, and Further Maths is usually very desirable. You would probably struggle to get into a top rate engineering course without it. Thus, if you are to take five subjects, your best option would be to choose Maths, FM and the three sciences. This is one of the easiest combinations of five subjects, given the overlap between them.
I don't see the harm in taking five. If, in the first week or two, you feel that the workload is going to be too great, you can always drop down to four. You can't, however, take up a fifth subject if you only start with four.