I think the Dalaman - Stansted or Stansted - Dalaman Pegasus flights should show after mid to end of september. If you book Dalaman - Sabiha Gokcen and Sab Gok - Stansted separately, not sure if your luggage would be taken through automatically. Also, not much time to check in again at Istanbul, if you have to do so,and luggage allowance for domestic flights is lower at 15 kilos.