Sloane - who played unhinged Niall Rafferty until November last year - takes on the role of Josh Hunt. Tointon - Walford's Dawn Swann - is to portray his screen fiancée Ami Ryan.
Meanwhile, David-Lloyd, who starred as the now-dead Ianto Jones in the Doctor Who spinoff, will appear as Jeremy Preston.
Details of the episode have yet to emerge, but it is said to revolve around a lovers' tiff.
"Since The Bill changed its format, I really wanted a part and am so happy to have been cast."
He added: "Kara and Gareth have been brilliant and we've had really good fun filming - we've had such a laugh."
The episode, which began filming last week, is due to wrap next week and will air towards the end of January.