Just a sleeper. You're going to want to bring a carrier and they'll be on you the entire flight so you'll both be warm. Bring layers for yourself!!
Bodysuit, Leggings, top and jacket maybe. Take a blanket cos it’s always chilly. Anything you can take layers off at the other side if needed
Something comfortable, that infant can easily be changed. Sleepsuit would be ideal. Also consider layers if it might be cool in UK and warm in turkey so may want lighter outfit or be able to mix up layers so baby is cool enough. Have fun!!