Do you like paying more for less when flying? We don't.
You can pay up to €6 and more for a small 5cl bottle of Whiskey, Baileys or Gin, etc. Quite expensive if you ask me!
Instead, why not buy a few empty 50ml bottles in the Pound Shop (or Centz if your posh). Before flight, fill them up with stock from your liquor cabinet. Each person can bring a max. of 10 mini bottles. All you need to buy is the Coke and even that you can buy before you board, saving you even more money! I hear you, it's not a cans of Heineken, but it's the next best thing.
If you're really stuck, you can buy your alcohol before boarding, saving you some. But if you want a nice drink of whiskey on your flight, this is the only way!