Some thoughts:
So, there are signs indicating that parking with 2 wheels on the footway is permitted on both sides(the location is linked in the first post).
On your side, the sign is ahead of your car; on the other side, it's opposite your car.
As regards placing of signs, this applies:
When a sign provided for in the sign table in Part 2 [this is you] is placed to indicate the point at which a restriction, requirement or prohibition begins or ends, the sign must be placed on the road as near as practicable to that point.
And there you have it in a nutshell.
'As near as practicable' does not mean the actual point at which parking relief starts, it means what it says. The practicability point is taken by many councils to mean pre-existing columns. The problem comes with CEOs who interpret the location of the sign to mean that relief starts at that point and not somewhere near.
IMO, you have an arguable defence that the sign opposite indicates that footway parking relief starts there and not, as in your case, at the location of the sign ahead of you. If the sign on your side indicates the point at which footway parking relief applies then you are likely in contravention and the sign opposite is wrong and misleading. But if the sign opposite is placed correctly, then clearly you are not in contravention.
In short, the authority must recognise that the position of the signs is not determinative and that only by reference to the council resolution approved pursuant to s15(4) of the 1974 Act would the prohibited and permitted footway parking areas be determined. They must therefore refer to this before responding to the challenge.
I would not go near 'I have parked here before umpteen times without issue' because you don't have objective evidence. I would stick with the hard facts and the law.