Regarding flight plan filing
CAP694 CH1 Para 2
http://www.caa.co.uk/docs/33/CAP%20694.pdf
As per post above #63, flight was within uk airspace so
not required. A109 from memory, 'loaded' weight of less than 5,700kgs, so
not required there either.
It is
advisable that a plan is filed for a journey over water where distance exceeds 10NM from UK coast, and remote areas where Search and Rescue could be difficult.
In my experience it's
not unusual for a VFR flight UK-NI-UK to
not file a VFR plan. The rationale for those decisions should be another thread. Post #17 indicates that he probably booked out his departure, and his destination knew he was coming and when to expect him.