Firstly, well done to the pilot for getting the aircraft down safely, but it raises several serious questions. Regardless of whether it was a single or twin engine machine, the bottom line is it would appear the weather was way out of minima. These conditions were no surprise given the recent prolonged rain and humidity - he didn't just fly into this from clear weather. Having an IR would have been of no help flying into a private site such as Pennyhill Park (or even Battersea Heliport). An IR does not give you Superman laser vision. If he did have an IR, however, then given the adverse weather, why not fly an ILS approach into Farnborough, which would have been a far more prudent decision, even prior to the 'engine failure'? One can't help but feel that this was an [inexperienced] pilot whose judgement was marred by the commercial pressure of carrying such a huge celebrity. With the greatest of respect to the great Dame, her soul is worth no more than any other fare paying passenger or, indeed the pilot's own. Had this been a normal Joe Bloggs charter, would they have made the same decision to go ahead with the flight? Also begs the question, why he overflew/flew close to 2 airports; i.e. Farnborough & Blackbush? Again, I say well done to the pilot, but this could so easily have become yet another statistic. Engine failure aside, this was no night for flying into private sites.