During climb-out from Stansted Airport (EGSS), the flight crew experienced an illumination of the “Master Caution” indicating “Overhead” on the main instrument panel. A short time later, the “Master Caution” (Overhead) illuminated again, followed by an additional warning light on the rear overhead panel, indicating that the passenger oxygen masks had automatically deployed in the cabin. Having reached FL 143, the aircraft was descended to and levelled at FL 100, where further analysis by the flight crew determined that the pressurisation system had not been properly configured for flight. The aircraft returned to the airport of departure, where it landed without further incident. There were no reported injuries to passengers or aircrew.