From the NATS Air Traffic Manual part 1:
"A pilot may file a flight plan for any flight.
A pilot is required to file a flight plan:
(a) for all flights within controlled airspace, Class A-E, which are conducted in accordance with IFR;
(b) for flights within Class B, C, and D airspace conducted in accordance with VFR;
(c) when he wishes to receive an air traffic advisory service (Class F airspace);
(d) for all flights which will cross a United Kingdom international FIR boundary;
(e) for any flight where the destination is more than 40km from the aerodrome of departure and the aircraft's maximum total weight authorised exceeds 5700kg.
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 search and rescue operations would be difficult, or;
(b) if he intends to fly into an area in which search and rescue 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."
Hope that answers the first couple of bits, and filing a plan is done normally by completing an official form and sending it to the relevant ATC centres/aerodromes via the AFTN telephone network. Alternatively, plans can be filed by telephone directly, in an ATC tower (called booking out) or even just over the RTF by calling the relevant agency.
As for the danger area bit, I haven't a clue!!!!!!!!!