Police already have dash-cams, but when something suspicious happens they either don't release the footage or they claim that it malfunctioned. Similarly, the London trains had cameras on them on 7/7/2005 but the authorities stated that all the cameras malfunctioned.

That problem is easily dealt with, though: if recordings should have been available under normal circumstances but weren't provided, create a presumption to find against the authority/officials who should have been on camera unless there is compelling evidence that the lack of recordings was due to a genuine problem.

Officer accused of police brutality and has no recording without a good reason? Go to jail. Suspect accused of resisting arrest and officer has no recording without a good reason? Automatic acquittal.

These kinds of devices can be quite robust, and it's unlikely that only a single recording device would be present at the kind of incident where this sort of issue might arise. The odds of all of them failing at the same time are negligible under most circumstances. And the idea that it's not possible to detect those failures and recall the affected personnel to replace their equipment is similarly implausible under normal conditions.