You need to check what you are being asked.
Firstly, a SAC (or indeed a Fixed Penalty) does not count as a conviction. So if you are being asked about convictions and FPs only, a SAC does not have to be mentioned. However, if you are being asked about convictions, FPs and SACs you need to find out whether you need to mention those you have already done (presumably none in your case) or those you have done and/or those pending. Furthermore, many insurers now ask you to declare any convictions, FPs or SACs which occur during your policy period (i.e. any you are penalised with between renewal dates) and not just wait for renewal. You need to check all these things.
On a general note, whilst you may get away with obtaining a policy or a renewal without making full declarations you will not if you have an accident - particularly one involving personal injury - that costs a few bob to settle (which is, after all, the very point of having insurance). You may find yourself uninsured if you have made a false or incomplete declaration. So a good rule is “if in doubt – declare it”. It may cost you a few bob extra but it will save you a shedload if you have a shunt.
On another general note I assume what you have received a “Notice of Intended Prosecution” (NIP) asking you to name the driver at the time of the alleged offence. A speeding action - whether dealt with by a SAC, a FP or inn court - cannot proceed until this has been completed because the authorities do not know the identity of the driver. NIPs are usually accompanied by a conditional offer of either a fixed penalty or a SAC if either is appropriate. If you have not already done so YOU MUST complete and return the request for driver’s details within 28 days. If you do not you will be charged under Section 172 of the Road Traffic Act (Failing to Provided Driver’s detail). This is a more serious offence and carries six penalty points. If you passed your test less than two years ago six points will see your licence revoked. You will revert to provisional status and have to take both parts of your driving test again. Do not wait for 95 days if you have not done this.
Jennyjoan – If you’re not a driver you’re excused. But if you are I’m appalled. You need to read Section 5 of the Highway Code.