Images from dashboard cameras can be used in legal proceedings but their evidential value all depends on the quality of the images and what they show. Just because a camera has captured an incident does not automatically mean that the police will be able to take action in every case – it will all depend on the circumstances.
Some police forces allow you to submit evidence captured on dashboard cameras via the internet, whilst others don't. It will all depend on your local police as to whether they provide this service. If they don't, you would need to contact them via the non-emergency 101 number for guidance.
In any case, it's important to remember that whilst a dashboard camera may be recording someone else's driving, it's also recording your own. Camera footage that shows that the driver doing the filming is using a mobile phone, driving aggressively or committing other traffic offences may result in them being prosecuted as well. Additionally, footage that reveals loud music blaring out, bad language or an aggressive attitude will also affect a court's decision.