I always wear cotton trousers (I have a great pair of combat style I got from Gap a few years ago so a few pockets).
Over that, I wear a plain white or black t shirt, a cardigan to match trousers and have a mobius scarf or a wrap around my neck. Depending on the flight and destination - the scarf will be just to cover my eyes (like a hoodie over my head) and help me sleep; or be a warmer one as an extra layer.
On overnight flights, the tshirt may be long sleeved as I always feel cold trying to sleep. Or have an extra layer (tops I can layer, or even an actual hoodie to go over the cardigan for sleep).
I would always have a change of clothes in my carryon anyway, for emergencies, and would be likely to change my top in the morning so having a warmer set leaving UK and a more summery version for arrival works well. And by having both sets in the same colour, they can work well with the trousers/cardigan combo. (So a long sleeved white top leaving here, and change to a scoopneck vest-type tshirt for arrival perhaps).
Comfy shoes, my long haul flight stockings (oh so sexy, but really DO help keep swelling down), and a comfy pair of socks over those (I sometimes put in a pair of hiking socks for sleeping and change back to regular thin socks for shoes) completes the look.