Working Hard, I do agree with you that being able to go to Cherbourg and do a touch & go just negates the whole process. But I still think that there needs to be some sort of notification even if its retrospective.
You see, I reckon that if you made a habbit of flying to Jersey via Cherbourg or wherever in France, then Special Branch would start getting suspicious. Then you may find a welcoming party either at Jersey or home base or even at your house! So I think the mere fact of having to be honest and tell someone what your doing can weed out the dodgy geesa's! BTW I am not suggesting you are one of them, I am just making a point!
I think someone else came up with the real reason SB are here and its all to do with transfer of money etc to the tax haven's. I really don't think they are there to stop terrorist attacks in light aircraft or to stop the dodgy person on their first offence, as they wouldn't be informed of the flight as suggested by people here. But it is a way that any abuse of the system can be collated and dealt with given time.
I remember flying a student of mine to Jersey, so he could meet his friends. He is wealthy. I could'nt believe how sniffy SB were being in Jersey. They knew of him and wanted to check his brief case. So I see there point!