You should definitely try to take also Further Math, as you should like and master Mathematics for an engineering degree. Otherwise you have to take Physics and Maths up to A Level.
Nevertheless you should really think wether you want to do economics or natural sciences or engineering, as they are while all linked, completely different courses. But you have still time and not being sure now, doesn't make you a worse applicant.
Work hard and rethink honestly your degree choice. As you already heard the Engineering course at Cambridge is a General Engineering Degree and wether it is suitable for you, you only find out by looking at it in detail.