Basically, you are not legally obliged or compelled to volunteer anything. If you are at fault and showed dash cam footage to your solicitor that proves it, they would not disclose it – nor is your solicitor legally obliged to do so.
If the driver of the other vehicle involved has knowledge that you have dash cam footage that proves you were are fault, then they can instruct their solicitor to order the footage to be disclosed – however, by that time the footage has usually been overwritten