Misleading advertising about vitality travel rewards

I’ve attached what Vitality advertises about the international BA flights discount. From this I deduced that a Gold Vitality member gets 30% discount on BA flights. I booked my return flight between London and South Africa (with a different airline) thinking I could get a discount for my local European flights using British Airways. When I try to book a BA flight with Comair (using my discount), I was informed that “all vitality international flight bookings needs to originate from South Africa in order to qualify for the discount”. That is not stated in anything that is advertised. And that is a very important condition to be excluded. As such, I’d like to lodge a formal dispute, because I’ve made my travel arrangements based on the information that is advertised, and such, budgeted with my 30% discount in mind. I still want my discount because even if the flight is not originating from SA, I’m an active vitality member in SA and as stated, “Vitality members save up to a maximum of 35% on flights (base fare) to 25 international destinations with British Airways.” And I want to fly to one of those destinations from London.