Out of curiosity, does a SAC appear on your license as a conviction just without points? Realistically how would they legally find out if you'd been on an SAC?
Attending a SAC is an alternative to the conviction rather than an alternative punishment once convicted. If you are eligable and opt for a SAC, you are never convicted of that (alleged) offence. This means that technically the crime isn't the same as technically the police take no action for the (alleged) offence as you are assumed innocent until proven guilty. Therefore no conviction = innocent as far as any third party (insurer infor example) is concerned from a legal persepective.
However, insurance doesn't work in the same way as the law and is based on statistical risk assesment. A history of speeding (perceived/proven/whatever) will no doubt increase the perceived risk, hence higher premiums.
Whether an insurer has the right to ask you, I'm not sure. You certainly have the right not to answer (but lying will ultimately invalidate your policy regardless of the question), they then have the right not to offer you a quote. Consider this though, if they have the right to ask you if you have attended a SAC, which is not a conviction, where does that end? Does that mean they can ask you if you've ever exceeded the speed limit but not being caught? It's effectively the same question afterall.