It is hard to imagine a more exclusive chain of prep schools than the one that has been entrusted with the education of the third-in-line to the throne. That privilege has been bestowed on Thomas’s, a group of four London public-school feeders, where the Duke and Duchess of Cambridge have chosen to send Prince George. With annual fees of about £18,000, Thomas’s, Battersea, is reassuringly expensive and boasts fittingly palatial facilities, including the Grade II-listed Great Hall Theatre, a gymnasium,…