Looking at GSV, there are signs for the Red Route at each end of Airport Way, so their service of a postal PCN is lawful.
Did you now know what Red Lines mean, as they seem pretty clear on GSV. Postal PCNs for Red Routes are money-for-old-rope, for councils as all they need are a few CCTV cameras, and bingo, the money rolls in. No need to employ expensive CEO patrols.
Frankly, I see no credible appeal argument. The irony of your situation is that if you had parked further in by the double-yellow lines they could not have sent you a postal PCN, a CEO on patrol would have had to serve it directly to you or the car.