If I may just emphasise something already said; YOU DO NOT NEED SB PERMISSION. The Act requires a notification only and a GenDec to Customs is not part of the Terrorisn Act 2000. Justiciar said a flight for hire and reward can be stopped by SB, on what grounds please and where is the authority to which you refer?
I had an occasion a few years ago when a SB officer attempted to stop my departure to IOM from an unscheduled stop at a major airport in the UK. He was unsuccessful in 5 minuts flat after one telephone call to his HO. Only on one other opccasion has it been a problem. I was outbound to Jersey and was getting nowhere with SB so simply changed to Le Touquet and then to Jersey. SB did not like it one little bit but ce la vie. On no other occasion has there been any difficulty so it just is down an idiotic (very rare) PC Plod.