By Michael Holden and David Milliken LONDON (Reuters) -British foreign minister Liz Truss clashed over tax policy with former finance minister Rishi Sunak on Friday, as the five remaining contenders to be Britain’s next prime minister went head-to-head in the first of three televised debates. An initial field of 11 challengers has been whittled down following two days of votes by lawmakers from the ruling Conservative Party. But no individual has yet emerged as the obvious successor to Boris Johnson who announced he was stepping down following a series of scandals. While Sunak has topped those…