By Elizabeth Piper LONDON (Reuters) – British Prime Minister Rishi Sunak urged voters and his own party on Sunday to “stick to the plan” for “long-term change” at an election he has signalled will most probably come in the second half of this year. After firing the starting gun on election campaigning last week by saying it was his “working assumption” the vote’s timing would be later this year, Sunak listed what he described as the progress he had overseen since taking office in late 2022. With his governing Conservatives heavily trailing the opposition Labour Party in the polls, Sunak is hop…