PAPI
I really dont mean to be rude, the more especially because your post suggests you have commercial experience, but I have to say I feel you are out of step with practical GA.
I regularly transit solent IFR and cant recall the last transit refusal. In fact last week I had a shock when they said no transits were being given. I made it "clear" that I really wasnt happy to descend 6,000 to scamper around the solent low level and miraculously my request was granted. I accept there are the occasional refusals but they really arent that common and more often than not because the pilot doesnt know how to reduce the risk of a refusal.
The reason for the fpl is to keep controlers in the loop to your location and intentions, especially in bad wx or water crossings. But I am talking a 4hr flight in a SEP.
I am really not sure about this concept of keeping anyone in the loop. Chances are on any non airways four hour flight in the UK a lot of time is going to be spent in open FIR. In open FIR you may, or may not, get some sort of service from a LARS provider but I have every doubt if you end up in a field in open FIR anyone will be scrabbling the search and rescue helicopter unless you get a Mayday away.
I agree if you have filed and dont turn up at your destination in the end they will initiate an overdue action which will include a trace of your known flight (to the extent for the reasons indicated above it is known). I therefore accept that if there is a risk you will be out of radio communications with an FI provider and therefore unable to get away a Mayday a FP has it uses (for example low level over the Scottish Highlands). Personally I would prefer to tell freinds or relatives what I am planning, give them a quick call when I depart and let them know I will call when I arrive and what to do if they dont hear from me - but then thats just me.
That is the only reason I would ever file a FP for an IFR flight in the UK remaining outside airways unless anyone can provide a convincing reason why I would derive any other benefit from doing so.