As we all know, Havering is a venal and rapacious council, but this PCN really does take the biscuit ! It would be well worth getting this into the local press to shame them.
Whilst they say they are not re-offering the discount if you submit representations, the discount period remains valid, so if you submit reps and have not had a reply before it expires you are entitled to pay the discounted amount. A case on this very subject was won at Nottingham when am appellant tried to pay the discount within the allowed period, having already appealed. The adjudicator was quite clear that the discount period was a lawful period for paying a reduced sum regardless of whether representations had previously been submitted. As I recall (it was some years ago !), the adjudicator also commented that it was a procedural impropriety to say that submitting reps forfeited the discount.
So submit reps on the basis of triviality of contravention. If they respond within the discount period, you are entitled to the discount. If they refuse it, take them to London Tribunals where you will win, and in all likelihood would be able to claim costs. If they have not responded, pay the discount before the period expires. Again, if they refuse it, they are behaving unlawfully.
Here are the relevant regulations: -
https://www.legislation.gov.uk/uksi/2022/71/schedule/2/made