So you parked on double-yellow lines that apply 24x7, and also part on the footway, which has been an offence in London since the 70s. This means the CEO had a choice of contravention to apply to the PCN, and chose the off-carriageway option because this has no exemptions.
So what made you think that parking partly off-carriageway directly upon double-yellow lines was OK ? Double-yellow lines require no signs, because they apply 24x7