You can usually get a cheaper chinese clone on ebay for about £60. These days they are just as good quality, just dodgy translated instructions LOL.
Theres loads of different systems out there. some clip on the mirror, some suckers on screen etc.
only id disagree with doaadi is the need for GPS. Its just not needed to prove anything. Our roads are well enough marked and unique to pinpoint locations from scenary and anything other than a minor scrape, your both going to be stopped anyway so the location is not going to be in doubt.
there are no legal evidentiary standards to be followed with these kind of systems. Even if you had an approved system the police use, installed in your car it wouldnt be considered the same simply because its not a police car.
but then video footage from civilian cars is almost always only needed for insurance purposes/fact finding of which any of the cheap HD ebay systems are adequate for that. As long as you can see the plate or clearly recognise the vehicle, no moron will get away with denying being involved.