The sign is clear enough on a normal approach, (turn left to enter the bay), and the sign faces the oncoming traffic because being a Red Route it's a no-stopping road. So if you approached in this way, you should have seen the sign and not stopped. TfL will refuse all reps regardless of validity, so you must be prepared to forego the discount and take them to London Tribunals if you think the PCN is unreasonable. I think it is, as you immediately drove off when you read the sign.
BTW, forget all about "I assumed vehicles can stop to load/unload between 7am-7pm". Never, ever, assume anything about parking in London or you'll end up getting a lot of PCNs.