Well the council doesn't know where you entered the zone, let's keep it that way and there is a fair chance they won't substantiate their position as to the signage. If the sign is there you can't say it isn't, but you're not required to incriminate yourself either (and it's up to them to prove the signs are there, you don't have to prove that they're not).
I would concentrate on the point identified by Pastmybest:
Dear London Borough of Croydon,
I content the alleged contravention did not occur for the reasons explained by the adjudicator in Rashidat Ahmad v London Borough of Brent (2190152145, 28 May 2019).
Yours faithfully,
Send the representation online and keep a screenshot of the confirmation page.