Its better to get a dedicated dash cam rather than overheat your phone.
"Some insurers (e.g. LV) ask when you take out cover if you have a dashcam and this reduces the price." I answer no to this, even though I have one. If you have an incident and your dash-cam just happens to have been damaged, stolen or just randomly didn't record (for example due to an SD card failure, or a blown fuse therefore no power) then what will happen? In the best case you could be asked to pay the discount the dash-cam entitled you to and in the worst case, you could face a claim of insurance fraud.
I suppose you could leave your phone permanently on charge while it's in your car, but with dashcams so cheap, why thrash your phone battery?
So you'll need to charge your phone couple times a day then? No thanks