The NPPL website states
here that you can fly to Ireland as-is, and to France with a proper medical. And to a load of other places.
So that phrase in CAP804 must be wrong. Clearly there is no territorial restriction, and probably the restriction on G-reg is equally somebody's invention.
In the ANO (cap393.pdf) the word "NPPL" appears only on page 289 and none of this is mentioned there.
CAP804 is not law anyway - it's a set of guidance notes.