It is 28 metres from the advance sign to the restricted street, so I don't think an adjudicator will consider this as too far away, frankly. So it looks as if the contravention is made out, but there could be a "technical" appeal based on their documents, website pages, and management of the enforcement process that could succeed at London Tribunals. The council have to obey the law same as you, and if they don't the PCN is void.
So wait a bit to see what others may come up with, but don't miss any deadlines for payment or representations on the PCN.