You need to file a GAR to the local Police unit of the area which you are departing with all details as on form there is no need for customs when departing Uk but you do have to give them 4 hrs prior notice when coming back, also police/sp branch need 12 hrs prior notice. When you arrive in CI's there is a local form to complete on arrival. I suggest you phone Swansea as they will give you the Fax numbers to file GAR. Some airfields(eg Wycombe/Fairoaks) require you to fax them the details and they pass it on to the interested parties
Also you have to file a FPL and check Notams as Guernsey has some parking restrictions and may need you to be handled at vast expense!!