I called Amex UK to enquire about what the programme meant, when booking an ex-ARN trip on EY; they told me it was valid only on ex-UK departures. The occasional discounts, however, are valid on (seemingly) anything – I took £200 off a £1985 CPH-DOH-SYD rt in J just the other week, with a "£200 off £600+ spend" offer on my platinum card.
It's not only valid on ex-UK departures, I've used it for QR flights from CDG quite a few times, also booked a one way BKK-CDG flight with it recently.