The 500ft rule allowed the pilot to come down to land for his diversion to Battersea but horizontal viz would have been poor and on a divert he may not have had that particular NOTAM to hand
Golf-Mike-Mike, I understand your sentiments but there is a statutory requirement to cover "any alternative course of action which can be adopted in case the flight cannot be completed as planned" although I agree in the case of an unscheduled diversion this might be somewhat challenging.
(Note am not saying Commander in this case did not comply with this requirement)
Quote from UK ANO:-
A commander must, before taking off on a private flight, an aerial work flight or a public transport flight, take all reasonable steps so as to be satisfied of the matters specified in paragraph (3).
(3) The matters referred to in paragraph (2) are that:
(a) the flight can safely be made, taking into account the latest information available as to the route and aerodrome to be used, the weather reports and forecasts available and any alternative course of action which can be adopted in case the flight cannot be completed as planned;