You cannot chose to remain silent. The purpose of the NIP is to find of who was driving. If you receive the NIP, you're obliged to declare who the driver was.
If you don't you're liable to points and a fine
Indeed. You have the right to remain silent upon suspicion of committing a crime, IE, being arrested. The whole point of it is to prevent you from saying something that would otherwise incriminate yourself which is why they read out a caution. Responding to the NIP is not admitting guilt, it is responding whether you were driving at the time, the key word is INTENTION which implies they will prosecute IF they have the right person. Remaining silent in this case is effectively refusing to answer that question so they will have to summon you to court to get the answer. You DO have the right to remain silent but it would be very stupid to do so, it means you will end up in court just so that they can figure out who was driving. If they can establish it was you and there are no other mitigating factors, you are stuffed.
They already know someone was speeding in that car, the CPS will need to be able to prove it was you should you refuse to answer. I would imagine if you did not report your car as stolen, refuse to answer whether it was someone else, that is you by default. If you instead accept the NIP and admit it was you, you can still contest the fine even after accepting you was driving through your own defence in court. If you say it was someone else on the NIP and have not reported your car as stolen and that person was not really driving you open yourself to a whole new world of problems really.
The only way to contest a speeding fine is to say you was not driving the car at the time or to later propose in court upon contesting the fine other factors as per your defence. The burden then falls to the CPS to respond to that. Ignore the NIP and you will just get a summons but the range of punishments in court available to a judge are a lot more severe should it turn out to be you at fault at the end of it.
The usual time to respond to a NIP is 28 days meaning after that you should expect a summons if you do not hear anything else.
Accept the NIP and hope for a speeding awareness course offer.