The lead initial detective inspector Brent Hyatt appears to be described in many reviews of this programme as a fictional character, in actual fact he is indeed a real policeman having been previously involved, amongst other cases, in the Serious Crime Units work on high profile so-called Honour Killings such as the case of Iraqi Kurd London student Heshu Yones in 2002/2003. This trial was also a landmark in UK legal history.
Near the beginning it is established that Aleksandr Litvinenko (David Tennant) lives in or near the north London suburb Muswell Hill. In Des (2020), the previous drama in which David Tennant played a historical figure, it was as serial killer Dennis Nilsen, who also lived in Muswell Hill.
David Tennant and Mark Bonnar have both appeared in Doctor Who: David Tennant as the Doctor and Mark Bonnar as Jimmy Wicks / The Eleven.
Detective superintendent Clive Timmons, played by Mark Bonnar, may seem a departure from Bonnars more usual Scottish brogue - in reality his London accent in Litvinenko may be the more genuine interpretation of the two as he now lives in the North London / Home Counties area of Hertfordshire
Actor Sam Marks, who plays DC Oliver Gadney, previously appeared in two other David Tennant productions. He appeared in the 2007 Doctor Who episode 'The Shakespeare Code' (playing an unfortunate character killed in the episode's pre-credits scene) and in an RSC production of Richard II.