Can I apply to become a officer before I finish year 13 with my predicted grades?

You need to be at least 17yrs 9mths to apply. If that's before you finish Yr 13 and before A'levels (or equiv) that's ok. You might however also wish to consider a university bursary or if you're not wishing to go to uni look at what you have done outside of school - part time job, volunteering etc etc. It's really important that you can show you have the maturity and life skills that will make you a good officer AFTER 44 wks training. The Army officer selection board is not looking for the finished product and actually the younger you are the more they feel you can be 'moulded'. I have seen a lot of 18yrs olds looking to join as an officer and many tend to get a Category 2 pass at the Army Officer Selection Board Briefing (AOSBB) which is a time delay (3 -12 mths etc) to gain some life skills, get fitter etc etc. I hope this helps and all the very best of luck to you.