Summary?
Is it safe to summarize this whole discussion as follows?
In anywhere but the UK, the FPL is used as a basis to initiate a SAR op when a plane is overdue. For this purpose, a flight plan needs to be submitted (generally to a FIO which distributes it further) and opened (when leaving from a controlled field, by the tower. Otherwise it's the pilots responsibility to pass the airborne time to the first ATC unit they're in contact with, who will then open the FPL.) The FPL is then transmitted via the AFTN to the arrival A/D, who will use the airborne time and the EET to calculate the arrival time. If a plane is overdue at this arrival time (presumably +30 minutes) they will initiate SAR action. Obviously, they will first ring a few likely places where the plane might have landed, and try to contact both the owner and the pilot, before the rescue choppers actually get into the air.
For this reason, it is very important to close your flightplan, and pass on any changes to the plan (for instance diverting to an alternate). Closing is done automatically at a controlled field. At an uncontrolled field, you ask the en-route ATC unit to close the plan for you when close to the field, or telephone the FIO after arrival.
In the UK the situation is different. Flightplans are not used automatically as the basis for SAR action. Instead, the pilot has to nominate a "Responsible Person". This RP is supposed to receive both a departure and an arrival message, and is supposed to initiate a SAR action if the plane is overdue. You can nominate your friend Bob as your RP, in which case you give him a call before and after the flight. But if you nominate an ATC unit (typically a "Parent ATCO") as your RP, you need to make sure they get these messages too, either by contacting them yourself, or making sure that the FPL ARR/DEP message is sent to them.
That's for flights which are wholly contained in the UK, or fully outside the UK. What for flights into or leaving the UK? As it's the arrival airfield which normally initiate a SAR action I'd guess the following course of action is the best:
For flights inbound into the UK, ring your destination before the flight, and ask them to be the Responsible Person for your flight. If you put them as destination in the flightplan (which you would obviously do) they should be getting the DEP message through the AFTN (or fax if they're not connected). And they know you've arrived, by looking out the window.
For flights outbound from the UK, all you have to do is file a flightplan and close it upon arrival.
Any improvements, remarks, modifications?