Born in Leicester, England, on June 26, 1931, Colin Wilson is pretty much the only notable British Existentialist, and is often considered "forgotten", despite the attention his books garnered at the time of their release.
While he wrote several books over his career, the most notable, by far, was The Outsider (Not to be confused with Camus' The Stranger, sometimes also referred to as The Outsider). According to the venerable source that is the back of the book, "The Outsider is the seminal work on alienation, creativity, and the modern mind-set". In it, Wilson examines the psychology and motivation of the Outsider archetype by scrutinizing the lives of those who he feels exemplifies it (Franz Kafka, Ernest Hemingway, Vincent Van Gogh, Fredrich Nietzsche, etc.).
Wilson is also the author of several other books, including The Mind Parasites, The Occult, and Religion and the Rebel, among others. However, each of these have been almost completely forgotten by everyone other than the most dedicated Existentialists.
As of the time of this writing, Wilson is still alive, and in his seventies (as anyone with basic math skills can tell from his birth date).