The MI5, like every other secret service, wants their members to feel part of a brotherhood and to never ever divulge any secret to anyone outside the group. They also want their agents to be utterly loyal to their superiors, and to trust them that any despicable thing they may be asked to do is done in service of some greater good, or that at least it serves their own interests.
All of these things together make it so that the immediate reaction to any apparent wrongdoing is to close ranks, tell nothing to outsiders like the police or prosecutors until some boss decides otherwise. And that boss will of course weigh any such decision against the risk that any minute secret might be revealed in an investigation, that any agent might lose a tiny bit of confidence, etc - and likely will brush it off and apply some paltry administrative penalty then move on.
All of these things together make it so that the immediate reaction to any apparent wrongdoing is to close ranks, tell nothing to outsiders like the police or prosecutors until some boss decides otherwise. And that boss will of course weigh any such decision against the risk that any minute secret might be revealed in an investigation, that any agent might lose a tiny bit of confidence, etc - and likely will brush it off and apply some paltry administrative penalty then move on.