The best thing to do could have been not disturbing them at first place, but I do understand thats the need of this question. So I wouldn't add an advise like "Do not disturb them at first place".

From what I have seen, Buzzards and other raptors (not specifically the ones you mentioned) would return to kill if left alone. But, provided that it is done quite early before they choose to go away. Many other domesticated birds would join in too, mainly ravens and crows. Raptors usually take care of the uninvited ones. 

In my city, Raptors have a typical hunting region where ravens won't happily go to the party. 

If its already too late, the poor fellow is anyway going to die in most of the cases. If it is so, **You shouldn't**:

 - Try to feed the prey if it is not dead yet.
 - Try to shelter the prey if it is not dead yet.
 - Try to shoo away raptors and ravens. Some things are supposed to happen, lets not disturb the balance of the nature.
 - Do not feel bad that you couldn't help either of them.