IMO,
Sadly, in the main London councils are ignorant about these matters but this doesn't stop them trying to apply their bespoke received-wisdom procedures.
But wrong is wrong.
The evidence - which as far as I can see is in pretty short supply on your side of the road- is:
Your car was parked with 4 wheels on the footway;
Opposite your car is a regulatory traffic sign which by law conveys the following meaning:
Vehicles may be parked wholly on the verge or footwayhttps://www.legislation.gov.uk/uksi/2016/362/schedule/7/madeThe sign does not allow councils to apply their own meaning, they've displayed the sign and must live with the consequences which in your case are that you parked wholly on the footway and therefore were acting within the permission granted by the sign.
So, why were you issued with a PCN?
It would appear that there's a belief in the council's mind that you were parked 'too far' onto the footway.
Tango sierra.
The sign opposite says you can park wholly on the footway without limit and not be in contravention of what is otherwise a London-wide prohibition on footway parking.*
So, what's the best way to deal with this, recognising that they won't want to admit that your interpretation is correct.
I parked as seen in the CEO's photos which unfortunately do not capture the permissive regulatory sign immediately opposite my car which for your convenience I have included**.
As the authority should know, this sign conveys the following lawful exception to the London-wide prohibition on footway parking:
Diagram 668 - Vehicles may be parked wholly on the verge or footwayhttps://www.legislation.gov.uk/uksi/2016/362/schedule/7/madeAs my car was parked wholly on the footway then clearly a contravention could not and therefore did not occur as I was acting in accordance with the permission granted by the sign. Consequently the PCN must be cancelled.
If I may try and pre-empt possible responses from you:
I was parked too far onto the footway: Cannot apply because there is no limit to the permission granted by the traffic sign placed by the council provided a vehicle is wholly on the footway.
My car was partially in contravention of the double yellow lines on the carriageway and therefore not permitted to be parked on the road at all, whether carriageway or footway*: Correct, but this doesn't solve the authority's problem because in such a case the only grounds available are '01 - Parked in a restricted street etc....' and not Parked on the footway.
*- don't do this again or you could be issued with a PCN for contravening the waiting restriction conveyed by the DYL which applies to the whole road, carriageway and footway.
** - make sure any photo captures the sign and where you were parked in one shot, make life easy for the challenged wee souls!