If she loves a doll at nursery, maybe get a doll which has an outfit (or 2) with lots of buckles, buttons, zips etc to open and close.
We've done the "wrap up cheap tat and dole out at a rate of 1 per hour" on longhaul flights. Works well for when a distraction is needed, spares can be used on the holidays or return flt. Only use them IF needed, don't bring them out just because it is another hour into the journey if DD is enjoying whatever she's into or it's mealtime etc.
YYYYY to having a change of clothes for everyone in your carryon bags - we travel a reasonable amount (DH and I for work, and we've done a fair bit of longhaul with DD as well) and you never know which flight will lose your hold case/hostess drop coffee/dinner will spill/someone in your party get sick or similar (or someone near you) - all sorts of reasons why a spare outfit, on board, is useful.
Colouring in - bring crayons so no sharpening needed or likely accidents with markers.
A favourite reading book to have a story together. (If she has a favourite blanket that's small, that might be useful).
A favourite teddy/toy to snuggle up to on plane and while away.
Have you been on a flt with DD before? If not, try and show her some photos of what airports are like (regardless of which airport you use, Manchester airport has a very good autism page - which explains about what the airport is like, noisy, busy, lots of people, need to do security etc - so great for anyone who has never been to see what to expect). Most airports have good websites, to see what is there. And look at the website of your airline too - show DD pictures of the type of plane you are going on and what the inside of a plane looks like.