Drawback form
If you just search HO60 fuel drawback form, you'll find answers on gov.uk site and appropriate form. Basically, you would leave UK with full tanks purchased at say, £1.58 and on return claim back the duty (38.35p per litre from memory). So effectively you have paid £1.20 per litre. Then fill up in Channel Islands with duty free fuel - there is nothing to claim back on this of course, but the best price is at Guernsey or Jersey if you are an AOPA member and show your card, an extra 5% discount is available.
Recommendation(strong) is that whenever you set off across water you start with FULL tanks - you may just encounter a reason why you have to turn round or divert elsewhere.
If you are going to venture to Channel Islands, why not have a great weekend at the Guernsey Air Rally - there are 41 entrants already?