Not a lot you can do if they ask you to cancel there flight plan.
And as well over 90% of VFR flights in the UK are conducted without a flight plan once your over the international FIR border its just business as usual for the pilot not to be on one.
Its better to get it done, than forget about it once you have managed to get a signal on the phone after landing. Because if you do all hell lets loose.
I really can't see it being an issue from a pilots POV