cars are already on our roads with factory installed dashcams, it's most likely already on the way even if the factory installed cameras aren't the ones used for parking, or it may be "added"
When it happens, it'll be entirely up to the customer to decide if they want the option at point of sale, but anyone who knows even a little about dashcams will most likely know they can get dedicated standalone dashcams or a combination of cams to set up for around £100-400 (currency may vary obviously) depending on how many cameras they'd desire, and importantly the features those cameras offer, as opposed to the dealers probably charge £1500+ as an "option" for maybe 4 cameras that may only record but might do almost nothing else (all the extra fancy functions most cheap enough dashcams can already do)
I have 3 cams, one in the front and one in the rear and a separate one I use in my work van or as an internal one in the car, total cost for all 3 was around £130 including two 32gb memory cards and one 64gb card.