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.