LONDON — After a stunning victory that shattered the Conservative Party’s 14-year hold on power, Britain’s left-leaning Labour Party moved to take hold of the reins of government. Incoming prime minister Keir Starmer promised to help Britain “get its future back” and claimed a “mandate to relight the fire” after difficult years of economic crisis and hardship. He will visit King Charles III at Buckingham Palace later Friday and be invited to form a government.