Maths

Depending on which universities you are looking for, i assume most will require maths but there may be a few which don't. If you have resits in September after the holiday you should go over maths during the summer to prepare for them and see how you do. Are you currently doing four subjects? If so, do you think you can spend more time improving maths if you drop one of them? If things dont go well for resits in September you can always sit the actual A-level exams and take a gap year. Engineering requires a lot of maths so i think it would be important for university applications. There are universities which only require Grade C (e.g. BCC), so as long as you are able to get a C in maths then you should be ok.

If you hate maths, engineering at most Unis will be a struggle - especially Mechanical Engineering.
What sort of engineering degree subject are you aiming at?