Wherever the final destination was is really beside the point, the first leg from Turweston to Chalgrove (with no IAP available) is looking more and more like a very dicey proposition considering the weather reported and observed on the day regardless of the equipment being flown and the vast experience of the pilot. If it wasn't incapacitation then I'm afraid this accident may be joining the many other similar reports from investigation boards around the world as your bog standard CFIT or spatial disorientation while maneuvering to attain a visual approach in IMC conditions.