The only protection against this in IMC is rock solid instrument flying skills.
That, and having practiced the maneuver correctly a few times previously. Looks like what was being attempted was a combination "obstacle clearance takeoff" and "instrument take off."
I note from the initial findings that the passengers were late. Granted, in a private deal, it may be that the discretion of actual take off time is in their hands. I'll edit this post when I recheck the time of sunset. The original take off time versus actual had more than one change in the environment.
EDIT:
The helicopter was scheduled to depart Gillingham Hall for Coventry Airport at 1830 hrs but the passengers were not ready to depart until around 1920 hrs. By this time, night had fallen and dense fog had developed; witnesses described visibility in the order of tens of metres.
From this language, my understanding is that the departure was intended to be prior to dark, though given the time of year, was it twilight at 1830?
I am reminded of a well publicized (on this side of the pond) accident in a privately owned aircraft. (Not a professional pilot at the controls). Initial flight was intended for Day/VFR, but the two passengers were late, by about three hours (pilot's wife and her sister) so they took off after sunset. Pilot (FW PPL) was not Instrument qualified, though he was working on it.
Ends up flying at night, hazy, no horizon, crash, ends in tears.
JFK, Jr. Late take off changed flight from day flight to night flight.
Back to this case, with two professional pilots to hand, one would expect that they re-briefed and adapted to the changing conditions ... or had that as their plan.