Presumably this is a simple case of deflection. By resigning, these guys are hoping that the prevailing opinion will be "well, they weren't proved to have done anything wrong and by resigning they have effectively punished themselves. So there's no reason to take this any further. Eh? What? Old bean? yes I'd love another G and T - what-ho!"
However, that ain't so. An innocent person would not fall on their sword. Two in as many days isn't "doing the honorable thing". It's a calculated attempt to minimise the damage - hoping that if they lay low, things will just blow over. Especially if the police can manage to drag their investigation out for 2 or more years. After all, you can't prosecute people while there's still an investigation going on - as the vested, establishment, interested keep telling us.
Maybe what we need is to put the investigation on hold. Prosecute some seriously deserving individuals, then restart the investigation to mop up the ones who were missed.
The tricky bit will be obtaining evidence. We found out from the De Menezes investigation that the police will all tell the same story (as they are allowed to confer before giving a statement - unlike ordinary people) and that they are willing to lie through their teeth to appear blameless in that story. We also discovered from the same investigation that it's highly unlikely that anything bad will happen to those involved. If killing an innocent person in cold blood doesn't give rise to jail time, what chance has a little police corruption got to punish the actual wrongdoers?
So, we have (so far) some high ranking officers leaving the force. We can't have any confidence in the police's ability or willingness to prosecute their own and we have an investigation that will nicely block any further legal action until at least 2013. On top of that, I suspect we have a cabal of politicians from all parties who have nothing to gain from turning over these particular stones, as whatever crawls out is as likely to bite them as anybody else. I would also expect that a lot of journalists and editors are also more than happy to drop this story as they, too, have nothing to gain and potentially a lot to lose from keeping it in the public consciousness.