My bad - assumed that the comment referred to the other envelope.
In that case, assuming that Royal Mail's refusal to honour non-barcoded stamps is lawful, it would appear that the OP failed to provide the information, by dint of using a stamp which Royal Mail had decided to expire.
As a general rule, we don't tell the OP what he should do - we advise him of the options open to him and the pros and cons thereof.
He can try to defend the s. 172, or he can ask for a plea deal.
For the former, plead not guilty to both.
For the latter, plead not guilty to both, but put in the mitigation section that he would be prepared to plead guilty to the speeding if the s. 172 charge was dropped. Conventional wisdom regarding the plea deal is that it had to be done by asking the prosecutor nicely on the day of the trial. Since the pandemic, the court system appears to have become more proactive in disposing of such cases without having to have the great unwashed in their courtrooms.
The next issue is whether the OP has a defence. There is a statutory defence if it was not reasonably practicable to provide the information. This would seem to depend largely on whether the court finds that the OP ought to have known or had reason to suspect that that the stamp was no longer valid. As the change was news to me, I have every sympathy for him in this regard, but I'm not a magistrate and won't be sat on his bench if it goes to trial.
Broadly speaking, if the OP has no reason to suspect that the notice has not been delivered, there is no reason for him to resend the information. However, if he does have reason, such as receiving a reminder, or a returned response, then the defence would not apply as it would be reasonably practicable for him to provide the information by re-sending it. AIUI, when court proceedings have been instigated it is too late to comply with the requirement.
The returned s. 172 response being delivered a day before the SJPN ought not in my opinion to be an issue. The fact that the SCU claim to have sent a reminder is a greater concern. Unless the OP can persuade the court on the balance of probabilities that he did not receive any reminder, any reasonably practicability defence would seem to be sunk. If he can, then it would seem to have legs.
