The probelm with a flight plan is keeping to it.
How many people actually fly the plan?
Lets say you route Goodwood, L2K, via LYD, level VFR, and there is a bit of weather on route. How many VFR pilots (and IFr for that matter) would give the obvious weather a miss, and then how many would duck around the low level CAS at Lewes?
The trouble in reality is many pilots do not fly to plan, and they would often be stupid to do so.
Therefore the plan may give SAR no more than an idea where to search - just ten miles left or right of track over mountains enormously increases the search area.
I agree filing should mean if you are overdue then at least overdue action will be initiated. However even that is not safe. Spend a day at my airport and see just how many times pilots are told AT does not have their flight plan (including for aircraft returning form Europe after a flight of a few hours). I hasten to add they HAD filed.
In reality there are not many areas of the UK where you will not establish two may communications for a Mayday - get the Mayday away, and make sure you always know where you are. Invest in a EPIRB, a mobile phone and a VHF handset and your done.
I know some of you are trying to come up with a really good reason for filing in the UK because that was the thrust of the earlier posts, but however much you try, there really arent any good reasons to do so, other than perhaps for those wishing to fly low level over some of the remotest parts of the UK without any other back up plan should things not go their way.