You may need to give up on the lhr-ist-lhr segments because the rules are vague on this issue. We all know that geography as defined by the fare rules are different from those defined by cartographiers. The list of countries that are restricted are all European plus the term Middle East. My guess is that Turkey falls under the Middle East region. Does make sense since all but a tiny portion is in the 'near east Asian' continent. Does not explain why many have travelled to DXB and IST on the same itinerary. Perhaps BA is more stringent on its interpretation of rules compared to AA or CX.