Technically, if it was not reasonably practicable to provide the information within the 28 days, it is a defence to do so as soon as is reasonably practicable afterwards.
To some people it was common knowledge that the post office reneged on non-barcoded stamps. The first I heard of it when the first time I saw a case of an s. 172 response not being delivered because the post office refused to honour a stamp bought legally and used in good faith (not that I have an opinion on the post office's action).
If the police will accept a late nomination it won't be an issue. If they will still offer a course, even better,
Personally, I would be minded to try to speak to the police (Fixed Penalty Support Unit/Central Ticket Office/whatever) before re-sending the response. If they are being helpful, they would usually put a note on the file, but they might suggest sending a note with the response. If you are having massive difficulty in speaking to someone, I would not delay re-sending unduly though.