If Chalgrove was indeed his destination then I assume he wasn't flying on a clearance which leaves making the flight as you described as they only other option.
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.