As he was outside controlled airspace then no clearance is required. In the UK you may fly IFR outside controlled airspace subject to pilot qualification.
So (and this is pure speculation) the pilot might have flown above safety altitude for the first portion of the flight and then performed some sort of cloud break procedure.
As has been stated previously this accident could be caused by many different factors - pilot incapacitation is a possibility, for example
Agreed, that is why incapacitation looks like the most likely option as the MSA for that sector is 1400 and the official & witness weather observations were from between clag to the deck to broken 800, so no matter which way you shake it he would have to have descended below MSA in IMC to get to the bases if indeed there were bases to be had.