Opinion: For PRIVATE aircraft Mode S is an infringement of PRIVACY.
Opinion: Mode S should not be necessary for flight within UK uncontrolled airspace.
Opinion: Mode S does not enhance flight safety outside controlled airspace.
Fact: Mode S is not even required in the London CTR.
Opinion: If your aircraft will not fly within airspace requiring Mode S don't bother with it. Apart from the spotters, who is going to be interested in your reg and any other downlinked parameters? Save your money and stick with A+C for ACAS safety.
I agree that Mode S is an infringement of privacy and I have always been of the opinion that Mode S for the sake of ATC was mearly a means to identify light aircraft infringing controlled airspace.
Indeed, there are some in NATS who believe that the whole of the South East of England should be a TMZ for this reason.
But now we have the potential of cheap TCAS driven by cheap ADS-B out, surely there is a safety case for ADS-B out even (or more so) in uncontrolled airspace. I don't think you can separate Mode S from ADS-B.