From the evidence now published by the AAIB, it appears to me (other opinions are undoubtedly available) that for as yet unknown reasons the commander/handling pilot did not successfully transfer to instruments. The co-pilot obviously realised that things were going wrong and advised him twice (re: "Attitude" calls), but this was unsuccessful in preventing him from allowing the aircraft to adopt far too much nose down, which turned the climb rate into a high rate of descent.
I think we've already discussed the "150 metres RVR" requirement. Unless this is laid down in more clear terms, I still don't see RVR being a pertinent limit for private flights away from an airfield.