To state the obvious customs and police notification is (presumably) only required in respect of the in bound flight - no need to report you are going.
Interestingly it is also my understanding that if you go to the CI for example, then to some where in France, and return from France to the UK only the customs notification is required.
For what it is worth I "tackled" the police today.
1. The telephone number at Gatwick was not answered all day, although they tell you this is the one to 'phone,
2. The local constabulary confirmed the number at Gatwick was correct and had no idea why it was not answered (no offer to find out why not). I asked them to confirm the fax number worked - they said as they weren’t at Gatwick they had no way of knowing - even more helpful,
3. I then asked them if they knew the information on the back of the GAR was wrong. They pointed out this information referred to Customs not Anti Terrorism, so I pointed out they were wrong. They said in any event the GAR was put out by the Customs and if the form was wrong that was nothing to do with them, Good to see are various Government agencies working efficiently and effectively together.
4. Finally I suggested that it was often very difficult to fax the form, particularly if you did not have access to a fax. I was told all airports have faxes! Of course I patiently explained that you were unlikely to be at the airport 12 hours before. I was told they were working on a mechanism to send the information by email. I decided not to explain that presumably they had had at least five years to solve that one since emails became really popular, so I it must be a very complicated matter!
These chaps are meant to assist us in providing this information as efficiently and painlessly as possible. The system is long overdue for a complete overhaul, the guys who run the system clearly have no idea what they are doing and have no understanding what so ever of the needs of pilots operating light aircraft. It the commercial world they would have been fired.