Complex sentence: He is the current English Super Featherweight Champion and former British Masters Featherweight champion, and is probably most famous for his loss to 2004 Athens Olympics silver medalist Amir Khan via a T.K.O. Since which he has gone on to fight for both British and World titles.
Split into simpler sentences: He is the current English Super Featherweight Champion and former British Masters Featherweight champion. Barrett is best known for his loss to 2004 Athens Olympics silver medalist Amir Khan via a TKO; since this fight, he has gone on to fight for both British titles and world titles.