Hello,
I don't know the UK regulations, but generally a dashcam shouldn't obstruct drivers' view, so it's best to install it for example behind the mirror.
Depending on the viewing angle, it's best to install it in the center of the windshield. And it should be installed in the area covered by wipers

Because of that in my non-FIAT car (as I have wipers that go both from the center to the outside), it is installed in the left top corner. OK, some 10 cm away from it.
There are generally 2 possible ways of getting power - through a cigarette lighter socket charger and with a hardwire kit from the fusebox. Depending what you want (parking mode?). If the car is under warranty and you don't wanna mess with the dealership, I'd use the charger.
Then you run the cable all around the window behind the trim. In my Ducato I've managed to hide the cable (with the microphone and the GPS receiver cables) behind the roof trim without removing it. I only had to remove the A-pillar trim. For my other car, a non-FIAT, I managed to hide the cable also behind the A-pillar trim without removing it. Although it was more tricky to get the cable from under the dash to the bottom of the A-pillar (it gets from the charger to the back of the central console, then on the bottom of the dash over my knees to the side, and then top, to the windscreen corner).