I don't do English but for maths: do as much practice as you can. Do every past paper you have time for before the exam(s). Make sure you know the formulae you aren't given in the formula booklet.
Economics: know the definitions. Helps a lot with short answer questions as well as a great way to start off essays. Speaking of essays, don't necessarily write every one out that you see- plan it, then get a teacher to read through it and see what they say and what feedback they give you. Obviously write out a few fully (and time it so you know you can still produce it in the exam) and give some of those to a teacher as well.
It's definitely possible and I wish you the best luck!