If he wasn't instrument qualified, then the conditions on the day were pretty poor throughout the UK from recollection of the TAFs and from flying that day further north. I think at Dunkeswell the cloudbase will have been sub 500ft with poor viz, hopefully someone can confirm weather conditions there at the time.
An engine failure is one option but unlikely IMO and wouldn't have resulted in the high speed vertical impact unless the pilot had stalled it while gliding.
I think far more likely is the conditions quickly overtook his skill and experience levels (and qualifications) and he got disorientated and stalled it or lost control completely.
RIP to them all