Given that the weather was evidently marginal and changing, the point is that this person managed to get into deteriorating conditions and the visibility went below VFR and continued to deteriorate.
As pointed out IFR is NOT an option. You are in sight of ground although the visibility is worsening.
As pointed out - VIS below 1500 metres you MUST land. What are the other options?
(a) Do you go back the way you came with the prospects of the conditions being worse than they were when you came that way?
(b) Do you decide to go up into it IMC (I hope not and illegal)
(c) Land. Just so happened to be a large area available even though it is a school playing field blah blah etc etc.
Which is the least dangerous?
(2) Subject to paragraphs (3) and (4), it is an offence to contravene, to permit the contravention of, or to fail to comply with, the Rules of the Air
(3) It is lawful for the Rules of the Air to be departed from to the extent necessary:
(a) for avoiding immediate danger;
(5) If any departure from the Rules of the Air is made for the purpose of avoiding immediate danger, the commander of the aircraft must cause written detailed information about the departure, and of the circumstances giving rise to it, to be given within 10 days of the departure to the competent authority of the country in whose territory the departure was made or if the departure was made over the high seas, to the CAA.