Drawback of duty but not VAT is available for whatever is in the tanks when you leave the mainland. From Compton Abbas you can pretty much route direct to Alderney. If Plymouth military are working they'll usually give you a transit through D026 occasionally they won't in which case you can normally route either further West or slightly further East and talk to Bournemouth Radar.
Jersey control will take you from about 15 miles north of the zone boundary and hand you to Guernsey approach as you enter the zone. They'll pass you on to Alderney tower with a weather update and the runway in use.
I normally cross the Channel at around 5000 ft,
Look out for our TB 20 parked on the grass near the hangars :-)