I asked a fairly large LR dealer whether they could provide a quote for fitting a front and rear dash cam, and their reply was that they would not do the work as it may invalidate the warranty.

Anyone else heard of this, and is this common practise?

In fairness they wouldn't want to fit third party items for obvious reasons. I've had them remove them for me but I guess that's a different issue.

I've never had trouble with Halfords doing them and they've fitted seven cars for me in the past few years. And I suspect a lot cheaper than LR. 2016 SDV8 Autobiography
And several others previously.

I'd expect the dealers aren't allowed to fit any 3rd party products - though some will get windows tinted etc....

I expect if you asked them to fit a tow bar, steps etc that were not the LR product, they'd refuse.

Yes but with a security device and a tracking system.

You wouldn’t have to strain the imagination too far to contemplate an irate customer going back to LR to claim something was either fitted incorrectly and that’s why their dash cam or worse still the vehicles electrical system had been compromised in some way, and JLR being absolutely unhelpful. The blame game would be in full flow and it couldn’t end well......and for what? A hundred quid service job on an expensive car. Of course they will stay clear of this type of work. 2016 SDV8 Autobiography
And several others previously.