Sorry to say it, but the advance warning sign reduces the impact of any reps you may submit.
However, their video shows how venal and rapacious they are, as they couldn't be bothered to install the camera so that it showed you passing the sign at the entrance to the buses-only exit from the roundabout.
So I suggest you submit reps against the PCN stating that their evidence, (the video), doesn't show you passing any signs barring access to the street. Please be aware that there is no right to be reoffered the discount, so if they reject, they may not re-offer it. However, by not re-offering the discount, it becomes an absolute and total no-brainer to then register an appeal at the Traffic Penalty Tribunal. They will then have the PITA of having to prepare an evidence pack and prove you did pass a sign, plus they must pay the adjudication fee.
So, really, it is all about your attitude to risk. Here is what the Earl of Montrose wrote; -
"He either fears his fate too much
Or his deserts are small
Who fears to put it to the touch
To win, or lose it all"