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.
Good to know - thanks! I do like those £200 off £600 (Plat) and £50 off £200 (Gold) offers that seem to pop up once or twice a year -- makes the annual fee much more palatable...