The offence relates to section 22 of the RTA
https://www.legislation.gov.uk/ukpga/1988/52/section/22If a person in charge of a vehicle causes or permits the vehicle or a trailer drawn by it to remain at rest on a road in such a position or in such condition or in such circumstances as to involve a danger of injury to other persons using the road, he is guilty of an offence.
It covers things like parking on a blind bend or approach to a humpback bridge or right on a junction so that traffic has to risk a head on collision to pass because their visibility is blocked- by the car itself or the road layout and the car forces traffic into a potentially dangerous overtaking manoeuvre, or blocking the pavement so pedestrians (e.g. pushchair & wheelchair users) have to step into the road to pass.
The prosecution has to prove that:
(1) You left it there (hence the S172)
(2) The position/condition that it was left caused a danger of injury to others using the road.
(The "in such condition" part would cover not applying the handbrake so that the vehicle could roll, having something unmarked sticking out a long way, or a severe oil/fuel leak spreading out over the road)
I'm unclear on (2) whether a photo of the position would do, or whether they have to observe someone risking injury.
The FPN is £100 + 3 points.
A defence would be "circumstances beyond your control" like a breakdown that locked the transmission so it couldn't be moved until recovery arrive.