Just to expand a bit on Justin's reply:
You are allowed to reclaim the duty paid on all fuel in your aircrafts tanks on departure abroad from your final airfield in the UK. The current rates are 30.03 pence per litre of Avgas, or 50.35 pence per litre for Mogas. So, say for example you depart with 160 litres of Avgas, you can reclaim £48.04 (160 x £0.3003)
You have to sumbmit form HO60, available somewhere on the HMCE website (or I can email you a copy if you PM your email address) HMCE will then post you a cheque.
More information
on the HMCE website
Edited to correct duty rates (Thanks Champ
)