The CAA has stated that it will not mandate any form of transponder and most flying machines do not and probably never will have one. The CAA took its current position because it became convinced that it was not technically feasible to fit a transponder to most flying machines. No transponder = no warning = work on your look out. The same is also true of radios, most do not have one.
Rod, you keep banging on about this, but when you say "most" you are obviously counting all those parachutists with a lawn mower strapped on their back.
The vast majority of "flying machines" which actually pose a risk to what I would call a normal GA pilot (flying a powered plane at normal non kerb crawling levels, making proper use of available vertical space) can most certainly be transponder equipped.
Gliders pose a risk to normal GA but statistics suggest it is extremely small - even smaller than then very tiny risk of a GA-on-GA midair.