If you go to Ireland/IOM/CI you have to notify SB but if you go there via a landing in France you don't have to notify anybody because the UK has no exit Customs or Immigration.
This is quite unusual. Many countries, and probably most/all "3rd world" ones, have exit Customs, which is a real hassle because it can seriously restrict the departure airport. Switzerland for example has a number of such airfields but some are "Exit Customs on request" and you just fax a 6hrs' (?) notice, nobody ever turns up, and you fly.
So, if you have forgotten to fax SB, or PC Plod refuses to let you go (does anybody have first hand knowledge of such a case, and the full details??) then just go via France. You waste a bit of time but not 12 hours.
All this stuff is stupid. The IRA worked out the route about 40 years ago, I am certain.