My tips are basically take everything you can to make you more confortable.
Take several inflatable neck cushions. These are great for lumbar support and stopping that armrest etc poking into you after several hours. Don’t fully inflate so there’s a good bit of give in them.
Take food and drink - get a Boots meal deal or buy a Telegraph for the free water.
Take your own entertainment, be it a book or loaded iPad.
Facing 10+ hours on a plane with nothing to do/drink can be miserable and yes it has happened to me hence the advice.