I went out and bought one of the "best" and most "highly recommended" dashcams I could find: the Nextbase 522GW. 1080p at 60fps, or 1440p at 30fps. I figured it's almost a necessity in the UK with the amount of crash for cash, and god damned foreign drivers (tongue firmly in cheek - to be fair, though, I come from a country that drives on the left).
Not sure if my expectations were too high for a device that's ONLY function is to record video, but the quality of the clips is just plain kak - my phone does a better job. Unclear sometimes, lots of noise. I've tried playing around with the polarising filter, different SD cards in case the write speed was the issue, but I just can't get it as clear as the review samples on YouTube (you can see people's fricking eye colour in some of them). Of course, they could be review samples that have been sent out specifically, while they then just flog the real item which is subpar.
Whatever you choose, make sure you buy a compatible SD card (some insurers may not accept your video as evidence apparently if you weren't using a card designed for video). Something that is heavy duty - dash cams get hot and are constantly writing to the card, and then overwriting as the card gets full. Durability is very important.