I don't think that just because Freddie died Effy will move on from him and be with Cook in the Skins movie if the movie ever happens. I mean, Freddie's death doesn't change anything, because:
1) Effy CHOSE Freddie time and time again over Cook so it's obvious that the one she loves is Freddie and his death doesn't change a thing.
2) For CE things ended on "You're my friend" and "Take me to Freddie", for FE it ended on "I love you". Again, I don't see how Freddie's murder can change it.
3) It would be weird if it turnes out that Effy hasn't made any growth through the series and will jump into Cook's bed just because Freddie is gone. She was strong enough to reject Cook, because she knew he is not the one she needed. Freddie's death doesn't change the fact that she doesn't want Cook.
4) Maybe I think too much of Cook, but I believe that he did love Freddie in his own weird way and I cannot picture him trying to hook up with his dead best friend's girlfriend. Also Cook showed some growth in the end of S4 when he gave Effy Freddie's notebook.
5) We don't even know what REALLY happened to Freddie... but even if he stayes dead in the movie see all my previous reasons.