I think TK will move some flights from IST (business traveller, connection) to SAW (point to point, tourism) to have some slots at IST for expansion and new destinations.
Also there will be a new terminal at SAW and Sun Express (joint venture between TK and LH) increasing flights from SAW more and more, too.
IST-LHR is/was flown with B773. I think TK will need these a/c for their longhaul flights in the future and will fly with A321 etc. to LHR and p2p traffic from STN (or maybe in the future from LTN or LGW) to SAW.