In Scotland the accused(s) would have been charged with Murder 'Art and Part'. Irrespective of who commits the fatal blow or pulls the trigger, if the aim of extinguishing life is common between the group, then all are as guilty as each other. The burden of proof remains with the prosecution and the standard of proof is beyond reasonable doubt.
The distinction with Baby P and the Bikers, I presume, is 'INTENT'.
From the outside looking in it appears that all four bikers intended to have a rival gang biker killed. In the case of Baby P, perhaps only one person intended to kill either intentionally or recklessly. The other two just stood by and watched or knew what was going on but didn't report it. The bikers assaukt was a one-off incident. Baby P's injuries were sustained (unfortunately) over some considerable time.
Is that a half-decent explanation? If not, let me know and I'll go back to the books (no bad thing) for a refresher. It is an interesting question.