It’s the kind of office space even a discerning wizard would love. London-based postproduction house Mill Digital Media Ltd., whose credits include special effects for movies like Harry Potter and Gladiator, is opening its first permanent office outside of England in an airy SoHo loft. The Mill Plans to move into a 15,000 square foot space at 451 Broadway by May or June. The split-level office has a penthouse, 18-foot ceilings and 2,500 square feet of decks with sweeping views of downtown Manhattan, making it an environment where the shop’s special effects crafters can glean inspiration when working their own magic.”We must have looked at 15 to 20 spaces. This space was by far the most dramatic,” says Bruce Sinder, president of Sinvin Realty Corp., which represented The Mill and landlord Crale Realty in the deal.For decades, a textile company rented the raw space. The Mill, whose executives signed a 10-year lease, will produce music videos and commercials in its new digs.