(Reuters) - British infrastructure company Balfour Beatty Plc reported higher first-half underlying pretax profit and increased its annual cash forecast on Wednesday, buoyed by lower costs and higher margin projects. The company confirmed that its Balfour Beatty Communities(BBC) unit in the United States had appointed outside counsel to work with authorities and look into allegations regarding the processing of property maintenance work orders and related incentive fees at two Air Force bases,..