Bailey has picked up a hip injury, while both Melling and Witt have calf problems.
Back-rower Shane Rodney has also been ruled out for the rest of the season after having surgery on a neck injury.
However, there is a welcome return for youngster Dan Sarginson, who has been out of action with an ankle injury since the end of July.
Broncos head coach Tony Rea commented: “Dan has been very diligent in his rehab and got himself back for the final two games. He now needs to transfer that same energy into the game.
“It’s been a productive week and we’ve worked hard in the break. We’re looking to improve the intensity and performance on Friday.”
London 19-man squad v St Helens: B Bolger, Bryant, Caro, Channing, Clubb, Dixon, Dorn, Gower, Howell, Kaufusi, Krasniqi, Lovell, McMeeken, O’Callaghan, Randall, Rinaldi, Robertson, Sarginson, Wheeldon.