Daminc said:
However, on the other hand, police are trained observers.
Certainly, but that is not the issue here.
In ordinary violent deaths investigations (say, a woman found strangled in her home), the police officers are 3.persons not directly involved in the death.
The woman's husband's testimony (say that he was out in the garden when she must have been strangled) must in principle be regarded as less reliable than next-door-neighbour Mrs. Watson's testimony that she couldn't hear him in the garden although she ought to have done so.
Although a possibility exists that Mrs.Watson rushed over to her neighbour and strangled the woman when she saw that the husband went out in the garden, the standard attitude would be to regard the husband's testimony with more suspicion than Mrs.Watson's, since he should in principle be the one with most interest in being believed.
(If the police doesn't believe him, they might think he strangled his own wife..).
In the London case, the police officers themselves are the ones whose testimony should be examined with great care, because they face grave personal risks if their actual testimonies are shown to be highly inaccurate.
Not so with the innocent by-stander; there exist for them (typically, that is) few motives other than telling the truth AS THEY SAW IT.
Whether what they saw is credible is of course a matter of further investigation; however, we should not at the outset harbour strong suspicions as to whether their motives for telling their version are more complicated than simply to tell what they honestly believe they saw.
Such suspicions as to the POLICE VERSION cannot be as summarily dismissed at the outset of an investigation.
No "instant" conspiracy theory here; the official version is as riddled with holes as a Swiss cheese.