That's so annoying, sorry you have to do them all at the end of Year 13! Your subject choice seems perfectly fine to me because they're all 'hard' A Levels. If you're good at Maths and are sure you want to do Chemical Engineering I'd consider taking Further Maths (if it's offered) instead of Biology because it's more relevant to the course, but take what you think you could get the best grades in. Unfortunately Chemical Engineering is one of the really popular courses so they're likely to be stricter (but like I said earlier if your Sixth Form isn't great they may make considerations, or at least interview you even if you don't meet what they 'normally' expect). I don't know anyone doing Chemical Engineering, but I'll ask around and get back to you!
I revised differently for each subject so I'll list what I did below for you
Maths: I tried to break down all the things we had to learn into a step-by-step process, so I knew that for a questions on 'X' I had to do 'A' then 'B' then 'C' and I just had to put the numbers in. I tried to understand exactly what was happening at each step as well. I also refused to give up on a question until I understood how to do it (if I really couldn't work it out I'd look at a mark scheme for the answer, but still not move on until I understood why the mark scheme was right).
Biology: I'll be honest, I found Biology really easy so my revision mainly consisted of learning mark schemes so I knew my answer would be what they were after. Biology has a lot of content and it's mainly just about memorising it at A Level so if your memory is good you won't have a problem! Try to make sure you understand why something works because it makes it so much easier to learn. Also don't forget about the coursework (or controlled assessment, whatever they're doing now!). I should have had an A* in Biology from my exams, but I screwed up the last coursework
Chemistry: I had a huge problem with Chemistry unless it was related to Biology or Physics. I just tried to learn it and didn't worry too much about understanding it. I'd just learn that if the question is about 'X' then I have to answer 'Y'. I didn't understand what 'Y' really meant, but I knew it's what they wanted! In the end I did understand what most of it meant though
Psychology and Religious Studies: As you're not taking these I won't go into too much detail. I really hated Psychology. I didn't find it hard, but it was so boring so revising it was a struggle! I just learnt some studies and the concepts. With RS there were only about six questions they could ask for each exam so I just learnt the answers to those, my only issue was with timing.
I never used revision timetables and, apart from homework and coursework, I didn't revise until Easter (or Christmas because we still had the January exams!). When I started revision though I was pretty much doing it all day and I had the evening off. At the start I'd focus on the subjects I didn't understand, but I'd always have a day a week to look over things I thought I understood to be sure I knew them (or any days I just didn't feel like revising). After I felt I knew most of my subjects I'd just randomly chose one to revise that day.
Hope this helps you!