This study confronts current influential theories that science fiction is either an American phenomenon or an international one. The study rejects the idea that British science fiction is distinguishable only by its pessimistic outlook - while also rejecting the idea that other designations, such as "scientific romance" or "speculative fiction", better fit the British product. Instead, the study traces the evolution of British science fiction, showing how H.G. Wells synthesised various strains in English literature, and how later writers, conscious of this Wellsian tradition, built upon Wells's literary achievement. An introduction defines what might reasonably be placed under the heading "British science fiction", and why. Chapter 1 examines previous critical ideas about the nature of British science fiction, revealing that most of them are based on untested assumptions. Chapter 2 explores the significance of the dominant motif of the island in British SF - a motif that suggests that British SF and mainstream English literature have been long and fruitfully intertwined. Chapters 3 and 4 deal respectively with British disaster fiction before and after World War II. They focus on why British science fiction has so frequently seemed obsessed with catastrophe. Chapter 5, a polemical conclusion, deals with the future of British science fiction based on its current predicament. "Ultimate Island" forms a theoretical counterpart to the author's "British Science Fiction: A Chronology 1478-1990", which defines the historical scope of the field.