Dash cams are best to show that you are NOT to blame.
If you are involved in an accident at a cross roads, it will show that you did not jump the red light as claimed. They also protect you from bogus claims of people who say you hit them (for an insurance payout) when you didn't even touch them.
I am not sure how this video evidence would stand up in court. I don't think it would. I am pretty sure that in the UK it cannot be used as primary evidence.
For speeding it can't really, as there are too many variables, unless your speedo reading and apparent speed is way over the top.
However, in most cases once a video is shown in court, most people will not bother to contest the evidence, but for the police it is not the primary evidence.
Doing 150mph on the M25 on youyube, red light violations would still need me to admit guilt (primary evidence).
As for youtube. If I contested it, you could only really prove what stretch of road it was and that it was uploaded from my computer. Did my kids upload it? Was it my video? Was it my car? Was I the driver? Was it on the date and time shown? Was that my voice?
For insurance claims, it is usually the insurance companies who decide amongst themselves who is to blame. Which is why it is better not to be hit by someone insured by the same company as you, as they are often settled as 'knock for knock'.