Send Clowns, to answer your first question, It's because the UK Rules of the Air Regulations (Rule 20 to be specific) says
Without prejudice to the provisions of rules 27 and 31, before taking off on any flight from an aerodrome in the United Kingdom, being a flight whose intended destination is more than 40 km from the aerodrome of departure, the commander of an aircraft of which the maximum total weight authorised exceeds 5700 kg shall cause a flight plan containing such particulars of the intended flight as may be necessary for search and rescue purposes to be communicated to the air traffic control unit notified for the purpose of this rule.
Unless I am mistaken Chilli's usually good gen appears to be in error in its detail this time although the principle is correct.