it’s only when you are left with 1 paper in F , then you can opt to do the P’s in the Essential level provided you take the last F paper too in the same sitting.
I sent an email to ACCA connect and they gave me these options to sit :
1. F7 alone or 2. F7, P1,P3 or 3.F7 and P1 or 4.F7 and P3 But cannot do Optional level papers yet.