Here’s what the law requires those involved in a collision to do. This is non-negotiable irrespective of where the driver might think responsibility might lie.
Note, these details ought to be volunteered and ought not to have to be dragged out of a driver who’d probably like to depart leaving a mobile number at best. If the driver is not cooperating, call the Police, assuming that medical and physical circumstances permit. If physically damaged, to the extent of requiring medical attention, then call 999 anyway.
Technology being what it is these days, a swift photograph of the location, car (including registration) and driver are never wasted.
Crash reconstruction depends on the post impact position of vehicles/parties involved. See sample sketch below.
Record as much detail as possible: what may have been said, witness details, subsequent journeys and a time line. Then call the Helpline to take it from there.