The answer is three. These musicals are Kristina från Duvemåla, Chess, and Aspects of Love. Chess (with Benny Andersson and Tim Rice) and Kristina från Duvemåla (with Benny Andersson) premiered in London, England. Aspects of Love (with Don Black and Charles Hart) debuted in London's West End.
The Melodic Journey of Björn Ulvaeus: How Well Do You Know the Swedish Musical Legend?