Havering have made their "no discount" decision public, because they are under an obligation to have regard to the Statutory Guidance of the Traffic Management Act 2004 so must be prepared to answer an appellant who claims they have not had regard to it. If Hackney are not re-offering the discount after rejecting an informal challenge received within the discount period, they would have to do similar.
SO far, I believe Havering are the only council so far to formalise their position. With postal PCNs, there is only one, formal, reps option and no guidance on discount. Certainly there is nothing at all about discount re-offer in anything based on the LLA & TfL Act 2003, which covers the majority of moving traffic PCNs issued by London councils. I think most councils realise that if they offer the discount, most people then pay, and it avoids them having to prepare an evidence pack and pay the adjudication fee to London Tribunals. For me, Havering are barmy, but there's a lot of barminess about !