This flight plan business in UK really is a shambles.
For instance, last week I departed a small but helpful airport in SW England for the Irish Republic having filed a flight plan with them. After departure, they called to say they were having problems with their telephone and could not open my plan. They suggested I asked nearby military controller (my next contact anyway) and ask them to open it for me.
After a while, military called back to say they could not find the 'frequency' for flight plans and could not help. Since I was approaching foreign airspace and by now far out to sea, I requested they contact my destination and ask them to open the plan.
The reponse was (I wish I could convey the incredulity in the controllers' voice) " You want us to TELEPHONE your destination in Ireland?" Me: yes please, if possible, so grateful etc.. Further miles flown, then "unable to comply .. telephone problems .. suggest you try London information." (In fairness perhaps the military are not allowed to make phone calls abroad and were trying to use a cellphone or something).
London information (limit of range by now) "No problem. flight plan activated by Heathrow at.. Phew!"
So here's a suggestion. Why not give London Information (and Scottish) an extra bod (which they desperately need at times) and make them reponsible for VFR flight plans? No more arguments about amending plans in flight, cancelling while airborne, etc.. and one contact for filing, activating and terminating.
Strangely, that's exactly how it works in the US (but then what do they know about aviation?)