This one has a level of complexity that needs careful consideration as to the definition of a "road" and "footway".
The key case is Clark v. General Accident [1998] 1 WLR 1647:-
https://publications.parliament.uk/pa/ld199...22/clarke01.htm (https://publications.parliament.uk/pa/ld199798/ldjudgmt/jd981022/clarke01.htm)
What must be considered is whether the alleyway has vehicular access.
If we look at GSV history there were bollards which prevented vehicular access in July 2016:-
https://goo.gl/maps/wZ3yS7SeTvbYsSbk6
Since then however they have been removed and a new building constructed to the southern side.
Another key point is that access to the alleyway has a dropped kerb.
The building at the far end of the alleyway looks like a substation with a loading door.
The other issue, as raised by the OP, is whether the place he parked in is not a road or footway but public land. This case is therefore relevant:-
http://forums.pepipoo.com/index.php?showtopic=90145&view=findpost&p=1347065
Given the references above, if we turn to the definition of a "footway" in Article 15 Greater London Council (General Powers) Act 1974 it states:-
“footway” means a way comprised in a road which also comprises a carriage way, being a way over which the public have a right of way on foot only;
https://www.legislation.gov.uk/ukla/1974/24/section/15
I would say, that if vehicles have right of way, we don't have a footway and the contravention given is a nullity.
Last, I would say the PCN (which we must see) cannot give Disney Street as the location because this alleyway is elsewhere.
Wait for other views.
Mick