Bar Shaker
The time that traffic is the most danger to me is on a summers day late in the afternoon flying back from Le Touquet around the north of the Heathrow zone NOT when I am in class A airspace.
And just who do you think the collision threat is from ?. Yes it is you the VFR pilot who as you point out are the vast majority of private pilots.
By 2008 a low cost low power mode S unit should be with us as these will also be caried by skydivers ! if the mandate stays as writen , the thing should be so light that fitting the mode S to an aircraft without an electrical system should not be to much of a problem.