I understand your concerns and I did ask the question myself "What if it broke during an impact and video wasn't available", the guy on the phone handed me over to a claims person who said if no video footage was available at the time and you can provide a picture of the camera in the car after the accident that would suffice.
https://www.axa.co.uk/insurance/personal/car/dashcams/
If you’ve told us your car has a dashcam, you must, where possible, send us video evidence in the event of a car insurance claim, as it will help us to investigate it for you and could settle the claim faster in your favour.
Whilst we won’t need to see videos from every trip, we may also very occasionally ask drivers with dashcams to send us a sample video to make sure that the dashcam is working correctly.