@NorthSouth. I am not suggesting the client is to blame for commercial pressure in this incident. Indeed as pointed out the client was suggesting not to take off from Redhill in the first place.... Twice.
By commercial pressure I mean in the mind of Mr Barnes for whatever reason real or imagined. Several radio messages and texts look to me to show a pressure to perform or at least be seen to try his best "at least we tried" "just put it down in a field if I have to" etc.
That ultimately looks to have led to Mr Barnes chosing to try to get to Battersea at his clients suggestion descending from a safe VMC position above the tops in an IFR machine into poorer weather below above central London.
As others have said and is so often the case this report has many alarm bells ringing throughout the chain of events.