Nigel, the only answer would be to make GA flying so overly restrictive that no one would be able to get airborne without fully briefing a supervisor or similar - a bit like the Military in many respects.
This clearly won't happen so it is up to education in flying clubs and from the CAA to ensure pilots are aware of the risks involved in flying in poor weather.
I know a lot of education goes on at the moment but the CFITs are still happening because the urge to reach the desired destination clouds (pun not intended) the judgement of many pilots and they selectively ignore the bits of the met forecast that don't suit their decisions. All the courses in the world won't stop you making a crap decision especially when you are under pressure from pax/friends to go.
Sadly the old lessons keep being relearned the hard way.