The fact of the matter is that "mode S" will only become a requirment if it is practical to fit.
At the moment the mode S transponders on the market are not practical for gliders , microlights and older aircraft without electrical systems , The CAA know this and have commissioned research program for a low cost low power consumption mode S transponder.
Untill such equipment reaches the market mode S for some aircraft will be inpractical and unenforcable in law.
The technical problems are not to large and just think of the advantages , you will show up on the TCAS of the faster moving traffic and if euorcontrol getts its act together even the light singles will have a form of TCAS and realtime weather radar images.
Twenty years ago who would have thought that a bit of kit that can place you to within 20 feet of any place on this planet would have cost under £300 and run on two small batterys , the mode S transponder will go the same way as the GPS and we will all look back and wonder what all the fuss was about.