It wouldn't be a waste of time at all
I wrote about 1000 words of it during the summer, and that was a really good starting point for me to talk with my supervisor about what was good/not so good and my ideas to continue. Don't worry about writing it wrong, if you've got a basic outline then your supervisor can point you in the right direction to improve it
It can be pretty time consuming, but if it's something you enjoy then it's pretty fun
Plus, doing independent research looks good on UCAS, especially if it's to do with your subject for uni. And with us, we basically did our presentations whenever we were ready - we just went to our supervisor and told him we'd finished, he looked over the plans and made an appointment for us to give the presentation. It might work differently at your school though, we didn't have an audience or anything, but at my friend's school they all did it in one sitting and the other candidates asked questions. With our school, just the teacher asked questions. But the point is, you might be able to give the presentation as soon as you're ready.
To be honest, the presentation wasn't that strenuous anyway - we were allowed plenty of notes and as long as you spoke confidently and answered the questions well, it was fine. Basically it was just to make sure we hadn't copied and pasted XD It did last about 20 minutes though. In any case, it shouldn't be too hard to do, even if it is close to exam time. Most of us did them around May, just after we came back from easter