Whatever you have sent and wherever you sent it, the police say they did not receive it. They are most unlikely to budge from that and their next step will be o prosecute you, so you need to be prepared for that rather than try to get to the bottom of it.
When you receive your court papers, it will probably be a “Single Justice Procedure Notice” (SJPN) rather than a summons. Hopefully it will list two charges: speeding and failing to provide driver’s details (FtP). You will be asked to enter pleas to those charges. You will then have a decision to make. You can either:
(1) Plead Not Guilty to both charges and then offer to plead guilty to the speeding charge if the FtP charge is dropped. Or
(2) You can plead Not Guilty to both charges and defend the FtP charge on the basis that you responded as required. Or
(3) You can simply plead guilty to the FtP charge.
The speeding charge cannot succeed without you pleading guilty as the police have no evidence that you were driving.
The usual advice in your circumstances is option (1). You can make your offer when you respond to the SJPN. In some areas that offer is deal with under the SJ procedure, other areas may require you to attend court.
If you intend to defend the FtP charge you will have to convince the court “on the balance of probabilities” (i.e. that they are more sure than not) that you did respond. Without any evidence of posting your response that may be difficult.
You haven't said but I assume you were the driver at the time the police are asking you about.