Have I missed something?
Are you wanting to file a flight plan for some particular reason or are you intending to route airways?
If you arrange your flight entirely outside class A you have no need to file a flight plan and will gain no advantage from doing so.
The only other time you would of course need to file is if the flight originated outside the UK in which case a FP is mandatory VFR or IFR.
Some might argue if your flight is in the UK and over "hostile" terrain or the sea it is worth filing to aid search and recovery others would disagree.