As regards the s. 172 offence
Subject to the statutory defences (reasonable practicability), the offence is committed if the required information is not provided within the 28 days.
Unless there is a contrary intention in the relevant legislation (s. 172 RTA 1988), s. 7 Interpretation Act 1978 provides that the requirement is satisfied when the information is delivered to the addressee's last known address, not when it is sent. There is no reason to believe that there was a contrary intention in s. 172, as it contains a defence that would prevent failure of delivery constituting a strict liability offence.
So, on the face of it, the elements of the offence are made out, and unless you have a defence, you are guilty.
That said, your tale leaves two obvious questions unanswered - why use DPD for the response, and more importantly what exactly do you mean when you say that it was delivered to a neighbour?
If your answers to the above do not change the position, the next thing to look at is the availability of any defences.
If it was not reasonably practicable to provide the information, and if you can persuade the court of this on the balance of probabilities, then you have a defence - which means even though the information was not provided (delivered), you are not guilty.
The question would appear to be what does and does not constitute reasonable practicability. Mostly, this would be for the court to decide on whatever facts they find.
At one end of the spectrum, if you properly posted the response and had no way of knowing that it had not been delivered, and no reason to believe that it had not been delivered, then sending multiple copies of the response just in case the first got lost would be beyond what is considered reasonable.
At the other end, if you posted the response, received a reminder saying that no response had been received, and then buried your head in the sand on the basis that you had satisfied your responsibilities by sending it and it's not your fault if they didn't receive it (which would be mostly incorrect), then unless you attended the same lodge as the judge, no court would find that responding to the reminder would have been beyond reasonable practicability or reasonable diligence - for the purposes of this statutory defence, there seems to be a fair amount of overlap between the concepts.
Obviously, you could have checked the tracking sooner. There are a lot of things that you could potentially have done, many of which would fall far beyond what could reasonably be expected. The question is whether or not that would apply to checking the tracking, and the answer is likely to lie largely in whether there was any perceived need to do so beyond idle curiosity.
Were you surprised not to receive anything before the SJPN after sending off the information?
As regards doing 'the deal'.
First question is whether the speeding charge is listed on the SJPN. Unless you have been charged with the speeding offence, you can't do a deal to plead guilty to an offence you haven't been charged with.
Not having a UK licence should not stop you from being able to do the deal - a 'ghost licence' would be set up for the points to go on.
