LONDON — Keir Starmer is off to China to try to lock in some economic wins he can shout about back home. But some of the trickiest trade issues are already being placed firmly in the “too difficult” box. The U.K.’s trade ministry quietly dispatched several delegations to Beijing over the fall to hash out deals with the Chinese commerce ministry and lay the groundwork for the British prime minister’s visit, which gets going in earnest Wednesday. But the visit comes as Britain faces growing