The chaos is already creeping into the balance sheet of large businesses like P.G. Rix Farms, which employs around 40 people some 90 minutes’ drive east of London. It grows mainly onions and potatoes, supplying industry giants such as McDonald’s and Tesco, Britain’s largest supermarket chain. It also plants sugar beets, cereals and willow trees, whose fibrous wood is used to make cricket bats. NBC News visited the farm on an overcast morning this Thursday. It sits just outside Colchester, which claims to be the country’s oldest town and was the Romans’ first capital in Britain. Today, the farm’s maze of tracks, rolling fields and water meadows are near a government-protected “Area of Outstanding Natural Beauty.” It is the kind of scene that stirs something deep in a certain English imagination: a landscape out of John Constable, the 19th-century Romantic painter whose work came to embody the nation’s ideal vision of itself. This is no mom-and-pop operation, rather an empire of alliums and tubers.…