Roffa
That could be a hot topic.
As you know for class D access (without crossing an international FIR) the FP has always been accepted verbally, well until recently when certain Scottish airports temporarily decided that they would prefer a FP filed before the flight. So to avoid any confusion, for class D crossing in the UK a FP does not usually need to be filed before the flight (but is "filed" when the airborne request is made). Prudence would suggest that for any flight over hostile / remote terrain or over any expanse of water filing pre flight may be a good idea although personally I have my doubts other than for areas where you know you may not be able to maintain a radio watch.