Quite sure that doesn't apply in a traffic incident, especially where no one is arrested. If you have to go to court to defend yourself because the other party in the incident decides not to take the blame, the dashcam footage isn't "previously withheld evidence", it merely backs up your version of events that you would naturally have disclosed at the roadside.
Generally, in the UK, you have to disclose evidence ahead of a Civil matter in a process called "discovery" and you can be penalised for not doing so. However, Civil cases take, months / years to come to Court and whether you use evidence is up to you. There's no requirement in a Civil case to disclose evidence that's not in your favour. The only thing you can't do at any point, is lie to the Court as that = perjury. However, generally a Court won't ask questions beyond the evidence presented.
If however, the other Civil party knows about the footage, they can subpoena it and use it to prove their case if it shows error on your behalf. That's a good reason for keeping quiet about it until after you've reviewed it. Although rare, cameras do lie, usually due to angle and foreshortening.
In a Criminal matter, I'm unsure if the police can seize dashcam evidence at a scene, S.19. 3 of PACE appears to give them the power to do so, although whether a vehicle constitutes "premises" is another matter. I doubt the inside is a public place. If neither apply, they'd probably need to revisit you with a warrant.
It can apply, you do not have to be arrested to be cautioned, you can agree to be under caution, things are not always done the correct way, I have had cases thrown out as the Police questioned myself without an appropriate adult, which seems funny when you're 51 years old but due to mental health issues they must use an appropriate adult from the appropriate adult service.
The police will caution you if questioning you about anything where they suspect a criminal offence has been committed as all interviews must be conducted under caution. They can question you at the roadside. If you are cautioned then my advice would be to simply state that if you are under caution, you wish to be interviewed with a lawyer present. At that point they'll either ask you to attend the police station voluntarily with a lawyer, or if they already have strong evidence of an Offence, arrest you, in which case, you simply request the attendance of a lawyer.
It's best to say as little as possible even when not under caution, because say anything that gives rise to a suspicion of an Offence, and they will simply caution you at that point, but by then you've already said something that's potentially incriminated yourself, although they will question you about it again under caution. Depending on what you said, it could be difficult to explain away.
I was in a prang where the police were called - I called them because the other guy was refusing to give me his details. When they arrived, nobody was cautioned, the police simply parted us to get our individual details & check them against their database. they weren't interested in hearing my accusation that the other guy came down the wrong lane & tried to force me off the roundabout - nor were they interested in hearing his accusation of my dangerous driving. Once they had all our details on their forms, they gave each of us a copy & went on their way.
I'm guessing there'd have to be some serious damage going on (or suspect drugs/alcohol) for the police to take any real interest.
It's an Offence to withhold details in the UK after an accident. Your name, address and insurance details must be supplied to the other party within 24 hours. The police made no arrests as the other driver clearly gave his details to the police.
Your last line probably clinches it, if there had been injury, they would have been interested in accusations of offences. However, when the damage is merely proprietary, the police often won't get involved as it's not worth the paperwork / their time for a minor infraction especially when they haven't witnessed it, as there's a lack of hard evidence of offences when it becomes he said / she said and UK Criminal Law requires proof beyond reasonable doubt.