Looking on the net Chicago Fire Season 8 is due on Sky Witness sometime in January 2020.
With a lot of these American TV shows there is a delay between the US air dates and the UK broadcast dates.
Some TV shows like The Walking Dead and Game of Thrones for example air during the same date in both the US & UK.
It all depends on what Sky and the TV Studios have agreed, where sometimes Sky will delay the start of a TV series to ensure continuous weekly airings thus avoiding the mid season breaks that a lot of these American shows tend to have or Sky are looking at a suitable slot in their TV schedule calendar when to air a show.
Chicago Fire Season 8 will be broadcasted on the live Sky Witness channel at 9pm on the 3rd of January 2020.
Where i would expect within a couple of hours of the weekly air date, each weekly episode will be added to the NowTV catchup service.