Where is the series available?
You can stream SAS: Rogue Heroes on the following platforms: fuboTV
MGM Plus Roku Premium Channel
MGM Plus
Spectrum On Demand
BBC iPlayer (free) in the UK: If you are in the US and want to watch SAS: Rogue Heroes on BBC iPlayer, you can use a VPN service like ExpressVPN4.
The search I did on my Roku TV listed MGM+ and Spectrum.