I think you have to use your best judgement.
If there was a multicar crash and you were early to the scene, you could pull around them and stop in front and go to check in the occupants but you are in danger if the traffic hasn't slowed because someone else could pile into it.
In cases like yours, it's often best to keep the flow of traffic moving and you did the right thing (including hazards). If you have a passenger, they should report it to the police so they can get a car out asap. You can feel a little... impotent doing this but often it's the best thing.
Rules 282 and 283 of the Highway Code seem the key ones.