Well, having now looked at GSV, and at their earlier views, this was obviously built as a very wide entrance to premises, but only about 1/3rd of the length was actually used as an entrance gate. However the main thing is that your car was off-carriageway, so in contravention of the London-wide ban on off-carriageway parking. This has been in effect since the 70s and requires no signs or lines.
So it looks as if the contravention is made out, sorry to have to say it. The entrance to premises is clearly no longer in use, but that doesn't alter anything. There may be a 'technical' appeal, so wait a bit until Hippocrates has viewed this thread, but don't, miss the deadline, (see PCN), for paying or submitting representations, unless you want to take the matter to the adjudicators.
