Aussie Andy,
I would guess that you are right as far as achieving the required addressing for the FP, i.e. which ATC units it gets sent to. A VFR FP doesn't get sent to any units en-route, as far as I know, except possibly the regional service e.g. London Info.
But for search and rescue purposes it's different, and SAR is the only reason I can think of for doing a FP within the UK.
However I suspect, as I've said, that flight plans with any unusual items on them get discarded without ceremony by persons abroad. Putting a phone/fax number on the bottom, even with the correct prefix etc, doesn't seem to make the slightest difference.