I'm new to this so check out some other responses as well. My understanding is that when you fly out of the UK you are effectively exporting the fuel you have loaded before the flight....so you're entitled to the tax back = drawback.
You can save even more money: there's another part of the drawback process which I'm told many people miss.....I didn't fully understand it, but it goes something like...you can claim up to a full tank's worth if you log all the flights since you flew out of the UK.
Does anyone have a better and more accurate description?