Original post by Doubtful JoyHmm, I'm not sure if I can entirely help with this. When I did my Level 3 EP in Year 13, I simply told my school I wanted to do it and they let me do so. Our school had dozens of people doing the Level 3 EPQ in Year 13 and dozens more in Year 12. However, I'll to give some tips that I think could be useful.
What you could do is show your enthusiasm at using the EPQ as an opportunity not only to gain "brownie points" in regards to a university application, but also as something you genuinely want to do to broaden your own horizons. Extended Project coordinators and university admissions staff always look for enthusiasm as well as achievements. For me, my A-level Biology teacher also happened to be my Extended Project coordinator, so I talked to her after several Biology classes and EPQ meetings about what topics/questions I was interested in doing in an EPQ on, why it excited me, what I could learn while doing it and how the EPQ writing was going etc. and really giving my teacher the sense that I was fully into doing the EPQ. Because of this, she was very supportive of me and my work and gave me flexible deadlines and other milestones during the course of the project. I imagine this could also help in an application to do an EPQ in the first place.
If you know your school's Extended Project coordinator and know you can approach them to talk about your EPQ aspirations, that could really help build up support for your application. Of course, everyone's circumstances are different so do make sure your EPQ coordinator is ok with personally discussing your EPQ aspirations before any written applications are submitted. It may be regarded as "cheating" if you personally talk to your EPQ coordinator in an effort to improve your EPQ application, but you won't know unless you ask.
Another thing to do is to really work hard on your written application form for the EPQ. Include why it would be useful to you (both for university applications and for personal education reasons), how and why you are excited about it, what you could and/or plan to learn by doing it etc. Make sure to corroborate anything you talk to your EPQ coordinator about in your written application, as they will remember the conversations you've had with them when they read your written application. By showing all the thought you've put into the application, you will improve your chances. Stuff like "learning to properly read and analyse journal articles" and "learning how to use a proper referencing style" (e.g Harvard, Vancouver etc.) are good generic plans/targets. Use them alongside more specific reasons for why you want to do an EPQ.
As a last resort, you may want to include the practical necessity of you doing an EPQ and getting a good grade. For me, I told my EPQ coordinator (and A-level Biology teacher) that I was planning on studying Pharmacy at university, which had a typical entry requirement of AAB along with good predicted grades and a successful interview. Doing a relevant EPQ would help me get an offer in the first place, and a good EPQ grade would help me get into the course even if I narrowly missed the offer requirements. If your Biology courses have high enterance requirements, you may want to make note of that in your application and discussions with your EPQ coordinator. They are more likely to let you do an EPQ if the course has high entry requirements because you'll simply need it more than others. Some courses even specifically say that a good EPQ grade would definitely reduce the required grade in other A-level subjects, and for those ones your coordinator is most likely to let you do an EPQ. For example, one of my offers said that if I got an A or above in my EPQ, my entry requirements would be reduced from AAB to ABB.
For reference, I got an A* in my Level 3 EPQ. I was just 1 mark off full marks. I did, however, write a 25,000 word EPQ, which was far in excess of the average EPQ length. I was only able to do this because I spent most of my free time in the autumn term of Year 13 focusing purely on my EP. If you are allowed to write an EPQ, make sure you balance how much time you spend on it with revision of your other subjects. EPQs should be considered a supplementary qualification, not a core one.
Hope I helped and good luck!