Complex sentence: When Beamish became a fugitive and fled England, Clarke became the head of The Britons, and formed with two others a splinter organization, the Britons Publishing Society.
Split into simpler sentences: When Beamish became a fugitive and fled England, Clarke became the head of "The Britons". In fact, he even formed and joined, with two others, its splinter organization, the Britons Publishing Society.