I think that the VFR flight plan system in the UK is unsatisfactory and needs to be put right. It is also a minefield for pilots used to the US environment where things work properly. IMHO, the only purpose that a VFR flight plan serves in the UK is a regulatory one.
For instance, I suffered under the nieve illusion for years that if overdue on a FPL to an unattended field, failure to cancel by phone would result in the launching of helicopters as it would in the US. Not so!! CAA leaflet 20A "VFR Flight Plans" explains the pathetic UK system whereby you are responsible for alerting SAR yourself if you crash through a home made 'responsible person' system! (or by crawling to a farmhouse).
Believe it or not, they actually assume that you have arrived safely unless you tell them otherwise!
Fair enough, if you land at an AFTN field then they will close the plan automatically or alert on overdue, but in practice this just encourages bad habits of not conciously closing plans on arrival, habits that can bite badly in the US. I believe that the pilot should be responsible for his own plan, not a controller who may or may not have received a copy of it.
London Information won't close or amend plans because as I understand it they don't get copies, unlike the wonderful US FSS who are actually responsible for filing, activating and terminating VFR flight plans under pilot direction. That's the real reason why you can't close a plan in the air in the UK, not the 'Duty of Care' that some controller once flamed me with for having the temerity to query this shambolic state of affairs.
It is commonly held that VFR FPL's are unneccesary in the UK, but the Welsh and Scottish mountains are just as unhospitable as those in the US, especially in winter. And many of us route 'just offshore' where a glide might or might not reach the coast.
I think that students should be taught to use FPL's on long cross countries and that renter pilots should be required to use them over hostile terrain by operators, just like they are in the US. But to do this we would first need a method to modify or terminate plans from the air to allow flexibility and at present this type of request just seems to meet with a refusal.
Someone queried recently whether a life had ever been saved in the UK by an overdue action and it's a retorical question because the inability to change the routing from the air, combined with the unfavourable weather that might cause you to change route in first place mean that you are unlikely to be on the track you originally filed. But in the US lives are saved by FPL's and it's crazy that we don't simply adopt their methods.