In the Met police area the form to be submitted is called Notice of Intended Flight. I think a Gen Dec. was an old Customs form pre EU days. If your airfield has arrangements in place you can depart from there.
Don't forget passports are needed.
Similarly notice of your return has to be given in advance of your Jersey takeoff. I believe something like 6 hours in advance, but check as it has been changed from time to time. Not required if you return via Bournmouth or other immigration manned airport.