I didn't say don't file a Flight Plan. All I said was should doesn't equal must.
From CAP493
10.4 A pilot may file a flight plan for any flight.
10.5 A pilot is required to file a flight plan:
a) for all flights within controlled airspace, which are conducted in accordance with IFR;
b) for flights within Class C and D airspace conducted in accordance with VFR;
c) when he wishes to receive an ATAS (Class F airspace);
d) for all flights which will cross a UK international FIR boundary;
e) for any flight where the destination is more than 40 km from the aerodrome of departure and the aircraft’s maximum total weight authorised exceeds 5700 kg.
10.6 A pilot is advised to file a flight plan:
a) if his flight involves flying over the sea more than 10 miles from the UK coast or flying over sparsely populated areas where SAR operations would be difficult; or
b) if he intends to fly into an area in which SAR operations are in progress. The flight plan should include the expected times of entering and leaving the area and the details must also be passed to the parent ACC. The ACC is to notify the ARCC concerned.
I don't see any 'shoulds' in there. Why should there be any 'shoulds' in the UK IAIP? The UK IAIP is the definitive publication. Either a FPL is required or it is not. Simple as that. The pilot still retains the option to accept advice. Should has a different meaning entirely and is poor guidance.