Very much doubt that you can clear Customs/Immigration into France by clearing into the Channel Islands, after all, the CI are independent states which have their foreign relations handled by the UK.
Your best bet is to fly into a French "Customs Aerodrome" - Le Touquet or one of the others listed in Pooley's and go on from there.
Don't - whatever you do - attempt to fly direct to a non Customs aerodrome without French Customs approval. They don't like it. Osman Durrani fell foul of them a few years ago and wrote about his experiences in Pilot
Can't help with the fuel drawback point. So far as I'm aware, you send the form the Customs at Southend, they send you a cheque.
Point to note though, they'll only give the rebate on fuel that hasn't already been rebated. That is, if your aircraft has been abroad recently and fuel drawback claimed, if some of the fuel on board has already been rebated that will be taken into account in calculating the amount you get.
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