We live in Spain and do the hop to Britain quite a bit! Hope some of this helps:
Try to time a nap for the flight itself, even if dd has grown out of naps by then try and really wear her out in the airport so that she is knackered and wanting to sleep. Noise of the aircraft will be the final nudge hopefully.
Wear layers. Airports can be really hot and planes can be really cold with aircon.
Take a towel in handluggage. Really good for blanket when sleeping, rolled up for pillow, propping over sleeping toddler to blank out bright plane lights, making tents and playing boo, dressing up, laying down to put toys on for quick gathering up, mopping up any sudden unexpected chuckups, and if major distraction is really needed, I take ds into the bathroom (only when there are loads, I leave the door open and we make way for anyone that needs to use it!) and we play in the sink. (He's at the loving water stage) When he was really small I used to use it for a swaddle too. If all else fails you can throw it over your own head and retreat underneath it with a gin and tonic.
I always take twice as many nappies as I would usually use in the same time on a non journey day, full pack of wipes, full change of clothes for ds and a spare T shirt and his slippers. Comfier in the plane but protects feet if he wants to walk in the flight.
I take lots of little snack boxes with different things in and let him graze, and try and time a hungry moment for takeoff and landing. Take a sucking cup or drink with a straw for takeoff as the sucking will obviously help with her ears.
I always have 3 bags with me! (I often travel back to see grandparents alone with ds while dh is working so have to make sure I'm really organised!) Main in flight bag has changing things, clothes, some toys, most of snacks, water and mints for me, towel. This goes in the overhead locker. I also have a large handbag which has 1 nappy, mini pack of wipes and some nappysacks, muslin, ds's spare T shirt, 1 large drink for him, favourite snack for emergencies and a brightly coloured plastic cheapy pound shop toy that he has never seen before, wrapped up. You can get a good 15 mins of distraction from it in an emergency and it doesn't matter if it gets broken or left behind somewhere.
Then I have all my travel docs in a bumbag.
I always try and wear him out in the airport before we board, so that he will be tired and clingy for the flight and easy to get to sleep!
My god huge waffle sorry, hope some of it is useful! Have a good trip