Just a quick read, the 'consultative document' suggests four options. Option 1 is do nothing, all the way to Option 4 which is
Option 4. Await the maturity of alternative technologies to Mode S SSR, such as ‘Automatic Dependent Surveillance – Broadcast (ADS-B)’, and implement them at a later date.
Funny, but to my reading of the doc the decision has already been made (Option 3), so I wonder what is 'consultative' about it. Maybe I am being cynical.
I think in order to get cheap kit, you need a massive market. I hate to harp on about it, but that means waiting for the US to decide what to do, and then you will get a massive economy of scale. Although Mode S groundstations are alive and kicking all over the US, I get the impression the US has decided to pass by Mode S for GA and is getting into true 'consultation' with the GA community with regard to ADS-B, Option four in the CAA doc.
Since ADS-B is a "cooperative" surveillance technique and 96% of the civil aircraft operating in the national airspace system are "general aviation" aircraft, the affordability of ADS-B is crucial to its success.
(And don't get too excited, that's the FAA talking)
So it would be a real bummer if you had to purchase and install Mode S kit, then the world moves to ADS-B, and you had to go out and buy some more new kit!