Outsource a niche model? No big deal.
Aston outsourcing a niche model is really no big deal.
Every car manufacturer in the world, from Bristol to Toyota, uses outsourcing. Outsourcing assembly of low-volume models dates back to the days of coach-building, and has recently been embraced by BMW, Cadillac, Mercedes (although its M-class factory in Alabama is its own, not outsourced), Fiat, Volkswagen, Porsche and Vauxhall, among others.
Also, Aston Martin has a long history of producing four-door saloons (starting in 1927), though post-war they've typically been badged Lagonda. The Rapide is built on the same VH platform as the DB9 and V8 Vantage.
Incidentally, Aston's V12 engines are cast by a German company (Mahle) and assembled in Cologne.