LONDON, Aug 1 (Reuters) - Travis Perkins, Britain's biggest supplier of building materials, reported a 31% drop in first-half operating profit to 112 million pounds ($144 million) in "challenging" market conditions, and said its near-term trading was expected to remain difficult. ($1 = 0.7797 pounds) ($1 = 0.7798 pounds) (Reporting by Paul Sandle; Editing by Kate Holton)