Sitting in his London studio among remnants of past projects—— angling globes, a plinth, a crib——British artist Mark Wallinger chuckles ruefully when he says, “There’s a glorious history of unbuilt things, isn’t there?” The topic arises because his most grandiose project to date, a commission to erect a 170-foot-high white horse sculpture in Kent County, southeastern England, is on hold as efforts to raise the £12 million cost flag.