LONDON — Britain is announcing one of the biggest shake-ups of its armed forces in decades, and it’s using the experience of the war in Ukraine as a model. The Defence Investment Plan, set to be published on Tuesday by Prime Minister Keir Starmer in one of his last acts before stepping down, shows the U.K. is copying Ukraine’s successful playbook to focus on “cheap systems destroying high-value targets and innovation cycles measured in weeks, not years,” the Ministry of Defence said on Monday.