Aside from typos the texts are identical, no abridgement or revision for The Lost World. Only the Oxford edition has a short forward seen in the 1912 editions. The second Pan edition has a one page biography of Doyle.
The four 1912 versions (serial and book in England and US) were illustrated but, not surprisingly, none of these editions have illustrations. There is a recent edition that does reproduce many of the 1912 illustrations plus others - The Annotated Lost World, Roy Pilot and Alvin Rodin, editors (Indianapolis: Wessex Press, 1996).
The art on the first Pan edition has faintly comical dinosaurs while the Pyramid and later Pan have the classic look of mid century dinosaurs - static and looking like toy models. The Pyramid also has dinosaurs too large relative to the humans in the foreground. The Harlequin has the most effective art of these early editions - it would easily catch my attention on a newsstand. The Oxford's art couldn't be more different with the now expected active, colourful and terrifying dinosaurs. All art is unattributed.
Pan 100 (1950)
Oxford University Press