You can claim for the duty at £0.377/litre for the total fuel in your tank on departure from your final UK airfield. As you are planning to land at Compton, the drawback will relate to the amount of fuel in your tanks on your departure there.
While Alderney fuel will still be cheaper, would advise against departing across the channel without enough fuel to make it back to the mainland UK in case CI/N France fogged in (it happens)