Imagine a scenario (which I plan to be in after the next exams session in June) where you have finished all fundamental level exams except for F6 and F7. Can you opt to do first F7 and P1 in December, and then sit for F6 the session after (ie. June)?
The only way that you can take P1 in December is by having already passed all the F exams, or be entering for all the remaining F exams in December as well as P1.