Maths is absolutely
essential for Enginineering! If you're serious about Engineering, you might like to see if your sixth form would allow you to take up AS Further Maths (you could teach yourself it) alongside your 4 A-Levels.
For example, I have an offer for Aerospace Engineering at Imperial which requires an A* Maths, A* Physics, and it also requires me to get an A in AS Further Maths. (And then an A in either Bio or Chem).
I'm currently taking Maths, Physics, Biology, Chemistry and AS Further Maths; the workload is tough but manageable, and I actually find it quite a fun challenge! In an ideal world I would have dropped Chemistry, as I don't like how AQA have done the second half of the CHEM5 syllabus... but that may cause Imperial to change my offer to require an A* in Biology as well, which - whilst attainable - would be a bit more stressful.
So, if I were you, and if I really didn't want to give up the Computer Science course (thought I personally think it may be for the best!) I suppose I would replace Chemistry with Maths (unless you fancy becoming a Chemical Engineer, in which case drop Biology instead)
and see if your sixth form will give you the opportunity to study AS Further Maths. All you really need is for them to enter you for the exams, and it's easy enough to teach yourself (I did the Edexcel FP1, M2 and D1 modules)! Good luck!
EDIT: As for what you could do
personally with Engineering, that's quite a tough one to answer (as I'm not an Engineer just yet!
) though I imagine the opportunities are quite endless! An Engineering degree will essentially provide you with a solid foundation upon which to
innovate - that is, essentially, solving problems with a combination of your ability to think critically, and the practical application of your knowledge of mathematics and the sciences. Sure, you'd be able to build a robot if you'd done some programming in your course! You would likely have done a few projects at uni, so when you go to uni you might decide to build a robot there! You could probably also help out a lot in less fortunate parts of the world - I suspect that there are a lot of charities who would leap at the opportunity to have someone with an engineering degree helping them out, working on various projects/overcoming any difficulties faced in those projects e.t.c.