I know the Miocene sperm whale Livyatan melvillei is known to possess teeth up to 36 cm in total length.

But I've read (on Dr. Richard Forrest's website) that some undescribed isolated pliosaurs teeth in UK were in excess of 40 cm (I've read even 45 cm).

Is that true ? Would that mean that pliosaurs are actually the predators that ever existed with the largest teeth ?

When I read your question, my first thought was the giant shark Megalodon, then Livyatan. These guys are definitely up there, and the latter definitely had bigger teeth than the former. I've not heard of pliosaurs with 40cm teeth, but I know Richard pretty well and I can tell you that he knows his stuff and is not one to exaggerate. So my inclination is to accept his big teeth -- which, yes, would be the biggest of any predator I know. Certainly bigger than in any predatory dinosaur, mammal, shark or mososaur.