I flew to London with Yasin when he was about that age.
Ask at check in to be seated next to empty seats and as soon as you can walk get your DH to get up and see if he can find an empty row. If you can get enough space so that your DD can lie down fully stretched out she'll sleep much beter.
Lots of small snacks are great because they will keep her busy. You can take fruit on the plane but you may have to dump it at your destination depending on its quarantine rules so only enough to eat in the sky.
Take a favourite blankie, toys, dummy (whatever comfort item she has) and a spare in case one gets dropped in the middle of the night because things move for miles under seats because the plane is always on a bit of an angle and crawling around under your fellow passenger's legs looking for a small object is really no fun.
Take a change of clothes for you as well as her because if she throws up on you or has a pooslosion you don't want to wear it all the way to Heathrow.
Throw away your ideas of travelling light - my cabin baggage bag is the biggest I can get away with these days. I put bags with-in the bag so that it's easy to find stuff (zip-lock is your friend) and put my stuff in the front pocket.
If she likes being read to take some books (a couple of old favourites and a new one too). One or two small toys are good. I buy a new toy in the shop at the airport because the search for the toy keeps my boys busy in the time between clearing immigration and boarding.
In the spirit of optimism I always take a book for myself just in case both the boys sleep and I can read it. On my last flight I actually managed it. We had 2 empty rows behind us so I grabbed them and lay a baby down on each and they went to sleep - best flight ever!
IMO a long haul journey is a good time to chuck some rules out the window in the interests of a peaceful flight for all.
You'll be fine.