There is no right in law to be re-offered the discount if representations are rejected, other than if the rejection is received within the discount period, then the discount is still available no matter what they may say. Under the 1991 legislation, there was no right to the discount, but most councils did re-offer it. When the Traffic Management Act 2004 took over, it was accompanied by Statutory Guidance. In the Act it says enforcement authorities "must have regard to the guidance". The guidance recommends that the discount is re-offered when refusing informal challenges.
I would add that in my opinion, stating that you are no longer going to follow the guidance because it's costing you money is a blatant disregard to it, but that's just my opinion.
Nottingham lost a case on the discount some years ago in relation to a bus lane PCN, where there is no informal challenge facility. The appellant tried to pay within the discount period having got fed-up waiting for a reply and payment was refused. The adjudicator ruled that the discount is a right in law, provided payment is made within its timeframe regardless as to whether representations had been submitted.