Whilst a few UK insurers are offering discounts, IME, they are way, way more expensive anyway - even after deducting their discount.
Some points to consider...
If they offer you a discount then, basically, they 'own' the footage. How will it look if, after a prang where the other guy claims you were all over the place, you cannot provide footage because your cam doesn't work? Mighty suspicious.
Also, I imagine the discount offered might be subject to conditions - "I've got a dashcam" 'ok, what make is it & how much is it worth?' "not sure of the make, it cost me £10 off ebay" 'you ARE having a laugh?'
Alternatively "I've got the very latest supercam 6000, it cost me £500 & it comes with gps, cpr, dvd, mp4, bbc, uht & ttfn" 'oooh, that sounds impressive, we could offer you a good discount however, the high cost of this unit would make your car a very attractive target for thieves'
Go for whatever insurance you would normally. When it comes to 'extras' or 'mods' declare your dashcam. My current insurer allows up to 3 mods without affecting the premium... "I have a dashcam" -'ok, what's the value?' "about £50" 'ok, that's no problem'.
In the event of a claim, providing footage is at my discretion.
In fact, in EVERY prang I've had, I've not only provided extended footage (showing events leading up to the event) but I have also taken stills from google earth AND google streetview.If streetview is a bit out of date, it's also worth driving the route at a quieter time, go a bit slower to make sure the cam picks up every detail - road signs, markings etc