Hi! First of all, congrats on finishing Y12, have a well earnt rest
I saw someone post that EPQ's aren't that much extra work, which I wouldn't necessarily 100% agree with. It completely depends on a variety of factors; who your supervisor is (you might know by now who teaches EPQ at your sixth form/college, but you can ask if you don't!) what your topic of interest is, what A-Levels you are doing, and how you can best manage your time. I did an EPQ (I have just finished Y13) and I am mixed about it; hopefully my experience can help you.
I wanted to do an EPQ because I wanted the independence, not appreciating that 2 of my 3 A-Levels required coursework which IS undeniably important, more-so than your EPQ if you do one. Please, please, take my advice; do not take on an EPQ if you have coursework in two or three of your A-Level subjects, because it is not worth it; coursework has a percentage which DOES go towards your final mark in a subject, and EPQs are a bit more murky.
I did mine on constitutional law. I flip flopped between something historical and something legal based; both of which were going to be intense as far as research goes, but if you find a subject that you live and breathe (aka you really love) and you have the time, I'd encourage you to go for it. Mainly because depending on your degree (if applicable) you will be doing a lot of 'coursework' in your own time, so you need to be able to manage time effectively, and be able to write long, detailed, intellectual essays. This is the main reason why I saw an EPQ through to the end.
I ended up having 2 supervisors; the first delivered a lecture in the early summer of Y12 about what it would require from us; I had an idea that I really liked, and he shut it down quite crudely. His criticism of my idea was completely fair, he just said that I'd need to establish a criteria for my judgement which would be hard to do, but he said it quite bluntly, and it set the scene for what he was like as a supervisor. I didn't really feel as though he helped me with my early ideas, and 85% of our class felt the same - we all eventually dropped out of the class, though most of all EPQ students in the year dropped out of doing an EPQ altogether; I think by the time we submitted an EPQ, there was only a group of like 8 of us, and we all fit into one class; whereas there were 4 full classes before Xmas.
The EPQ is as much work as you make for yourself, but my supervisor suggested that we have between 8-15 sources which had a direct impact on our essay (ie books, records, podcasts, things like that) and 5-10 more minor sources which we could use to weigh up our judgement. In the end I think I had something like 25-35 sources in total, including three or four that I had to translate; I am part Danish so I enjoyed translating the Danish sources that I used, but it was a headache.
The main reward for your EPQ is that some universities lower their grade boundaries for you, providing you get a top grade in your EPQ. Out of the five universities I applied to, three accepted an EPQ and seemed to lower their boundaries initially - though I got rejected from one of them, because my predicted grades were not high enough (even though they would have lowered the grades to AAA with an A in my EPQ, and my predictions were A*AA, so I don't know) so it is a weird process. It is, however, security that is greatly appreciated once you enter crunch time for your exams, because unless you have an unconditional offer, you need that leniency with grades, so my advice to you would be to take anything you can get. There is a lot of good to come out of an EPQ, so don't underestimate it.
TL;DR - If I was to give a percentage to how much I'd advise you take on an EPQ, based on what you've said, I'd say 60%. Consider 1) your subjects - do they allow for extra time to be taken on the side to do more work? 2) your attainment so far in your subjects. You're applying to Manchester and Edinburgh to I can see that you're doing very well, but you don't want to lose focus of your subjects, because they take priority. 3) is there anything that jumps out at you as to what you'd do for an EPQ? 4) does it link to your degree?
Everyone is different so nobody can tell you personally whether to do it or not. Just prioritize your subject studies, and talk to someone at your college/sixth form who is teaching the EPQ to get more advice from them.
Best of luck, and stay safe