I've quizzed my Honda dealer. Regarding third party dashcam fitting and invalidating my warranty, they ummed and ahhed for a bit then said it wouldn't be invalidated. Then they went on to say that if the car developed an electrical fault and a third party install was proven to be the cause, then they wouldn't be covering the cost to put the fault right. They were quite pragmatic about the whole thing. I don't think you can argue with that.
They're also happy to fit a dashcam that I supply. They said that it would cost £130-£140, though. They furthered with telling me that there's loads of people out there who would do it for less. They added, don't use Halfords, though. A bullet dodged there, perhaps.
They parted with a warning about buying a good quality one. They said some customers had fitted their own and it had "taken out" the reversing camera. They didn't elaborate whether that meant disabled or damaged. I wish I'd asked. How would a dashcam do that? Surely more likely a botched installation than an obscure side effect of the dashcam itself?