Why was a VFR or SVFR clearance issued (and accepted) in what was so clearly not VMC?
VMC is defined in the ANO as "weather permitting flight in accordance with the Visual Flight Rules" so is not relevant to this accident because it could not, legally, have been undertaken under the VFR. Only SVFR or IFR were possible. SVFR clearances do not require the aircraft to comply with the VFR - or indeed the IFR. AIP ENR 1.2 contains the slightly ambivalent wording "When operating on a Special VFR clearance, the pilot must comply with ATC instructions and remain at all times in flight conditions which enable him to determine his flight path and to keep clear of obstacles. Therefore, it is implicit in all Special VFR clearances that the aircraft remains clear of cloud and in sight of the surface." However the Rules of the Air Regs Rule (1)(k) clearly states that "special VFR flight means a flight...in the course of which the aircraft complies with any instructions given by that unit and the aircraft remains clear of cloud and with the surface in sight".
I think we can expect some clarification of what SVFR means in the AAIB report's recommendations, as well as such things as whether VMC on top is an acceptable way to transit the London CTR, what clearances ATC can offer into the London CTR, and who decides what the weather conditions are in the London CTR for the purposes of authorising transit clearances (in Class D zones, the ATC unit responsible for the airspace decides what the visibility is. There's no equivalent for the London CTR).
NS