A first-half brace of goals by full-forward Jonathan Maher helped London to a comfortable win at Ruislip.
The Exiles can now look forward to a league final, while Roscommon finish bottom of the table and are relegated.
The hosts took advantage of the stiff breeze and raced into a 2-10 to 0-1 lead after 21 minutes, thanks to Maher's goals and five points from Donie Reale.
Roscommon trailed by 2-14 to 0-3 at the break but were spared a humiliation because London added just four points in the second half.
The Rossies showed far greater urgency, with goals from Ronan O'Meara and Tomas Seale, but London had done enough.
Man of the match: J Maher (London)
Scorers – London: J Maher 2-2, D Reale 0-8 (7fs), K Walsh 0-3, S Frawley 0-2, D Moore, E Cooney, S Keane 0-1 each. Roscommon: Cathal Kelly 0-5 (5fs), R O'Meara, T Seale 1-1 each, S Curley, G Fallon (f), E Kenny 0-1 each.
London – D Russell 6; P Lane 7, B Costello 7, B Healy 7; D Bardon 7, B Regan 7, B Moroney 7; D Moore 7, C McAlinden 7; S Frawley 8, E Cooney 7, S Keane 6; K Walsh 7, J Maher 8, D Reale 8. Subs: PJ Rowe 6 for McAlinden (h-t), S Lambert 6 for Walsh (h-t), D Healy 6 for Lane (51), J Walsh 6 for Moroney (59), L Hands 6 for Maher (61).
Roscommon – P Dolan 6; C Shaw 6, I Delaney 6, S Curley 6; C Dolan 6, G Fallon 6, S Melia 6; Cathal Kelly 7, J Lawlor 7; A Murphy 6, R O'Meara 7, T Seale 6; C Egan 6, D Nolan 6, Cormac Kelly 6. Subs: N Connaughton 6 for Egan (27), E Kenny 6 for Murphy, L Kilcline 6 for Cormac Kelly (48), D Leonard 6 for Dolan (58).
Ref – L McAuley (Antrim)