Join Date: Nov 2018
Location: Istanbul, Turkey
Programs: TK Elite Plus,BAEC GGL,ITA Executive, AFKL Gold,QR Gold,HH Diamond,Bonvoy Gold,ALL Gold
Posts: 14,186
Update on the situation:
The reason why the BA Turkish POS doesn't allow double-stops on this itinerary was that it's limited to IST-LHR-XXX on transatlantic routes according to the routing restrictions on ex-IST fares.
Although, AA does allow it as they follow the MPM on ex-IST tickets.
By the way, I'm doing this trip with another airline but on my next trip, I'll probably prefer BA for the 4 segment requirement.