Well, if you are sufficiently aggrieved about it, remember you have the absolute right in law to take them to London Tribunals on this issue of signage. Of course you have to risk the full penalty, that's how the game works, I'm afraid. But first you have to submit representations to Islington, get them rejected, then register an appeal at LT.
If you submit reps within the discount period, they will very likely re-offer the discount; they don't want you to got to London Tribunals either, because it is a PITA for them, because they have to pay the adjudication fee, and also prepare an evidence pack.