(Reuters) - Comprehensive motor insurance policy costs in Britain rose 5% on average in 2019, the steepest annual rise since 2017, an insurance price index released on Monday showed.A comprehensive policy now costs 815 pounds on average, according to the latest index from price comparison site Confused.com in association with insurance broker Willis Towers Watson . The index climbed 4% in the fourth quarter compared with a year earlier.British motor insurers have been struggling with high costs,..