I was using Google Maps at the time, which directed me along this route from Blackheath. I’m really confused as to why Google Maps wouldn’t have flagged this restriction, suggested an alternative route, or provided any warning.
I call your case a "satnav-assisted PCN". These things are never up-to-date, and it is very unwise to rely on them in London and other large cities.
Can you please post a GSV link to the location. I note that the video doesn't show you passing a sign facing you, its for traffic the opposite way. At adjudication this sometimes wins because the council cannot prove the signs were there on the date of the alleged contravention. However, adjudication means the discount option is lost, so more of a risk.
On a positive note, it has been noted that Greenwich council seem to be in considerable chaos in their parking and traffic enforcement department, and have not contested some recent appeals at London Tribunals.Whether this is still the case, I don't know.