As a general rule most books sort of gloss over the transition from basal coelophysoid or ceratosaurian grade theropods to allosaurids, but megalosaurs are actually a fascinating bunch. They not only rose to prominence during the Middle Jurassic, but they continued to compete in the Northern Hemisphere alongside sophisticated allosauroids, and managed an impressive Cretaceous radiation (as spinosaurids). Sadly a lot of these specimens are not terribly complete, so if we want to restore them we need to do a lot of gap filling.
Torvosaurus is actually surprisingly complete once all of the referred specimens are accounted for (mercifully they also have quite a bit of overlap between material, so cross-scaling isn't such a challenge). So perhaps not surprisingly that's what was used to help fill out Megalosaurus and the new T. gurneyi.
Marshosaurus is too far from Torvosaurus to use it, so instead Condorraptor and Piatnitzkysaurus stood in. Also, to be fair to the megalosaurs and the completeness issue some of the more basal taxa like Eustreptospondylus and Piatnitzkysaurus are more complete, but I haven't got to them (yet!).