Chilly Monster is right, of course, about the drawback. Im afraid there is a lot of "duff gen" in the earlier posts!!
If you refuel you will ONLY get the "duty free" on the quanitity you buy and NOT the whole lot.
I suppose there is some convoluted way that you can claim the balance from HM Customs, but god knows how!
The simple way to get it all is, of course, to claim it all fom HM Customs. It is simpler than you think, and it always comes in a few weeks.
UV