GBALU53, the whole FPL thing was indeed part of my training and exams. The unit "The Full Flight Plan" is eight whole pages. Of these eight pages, two are "unit preamble" pages which contain no useful information whatsoever. Two pages (one of which is a map of the UK showing the difficult areas) cover WHEN you're required, urged and allowed to file a flight plan. Two pages cover the FPL itself, with the fields that need to be filled in, one page is revision answers. That leaves only one page to discuss HOW to file a flight plan, and what happens afterwards, including how a SAR operation gets started. I just read throught this page again, and nowhere does it mention the fact that the whole Responsible Person thing is strictly a UK thing.
Now consider the fact that the total stack of study books is over 10 cm. That I did my theory exams according to the CAA syllabus, had my practical flight training in the US, and fly in NL now. But even if you trained and flew exclusively in the UK I think you can get a PPL without ever having to fill in the ICAO form. Particularly if the school you flew with operated from an uncontrolled field, and the school dispatcher automatically assumed the role of RP for all flights.
I'd like to do this by the book, if only for my own safety, but unfortunately the books were not very clear in this case. That's why I came to this forum.