Two first half goals from the impressive Kevin Walsh and Shane Egan alongside points from Colm Quinn, Henry Vaughan and the accurate Kevin Finn put the visitors in the driving seat at the interval.
Three points in quick succession from Micheal Kelly, Kevin Conneely and Johnny Coyne suggested that a Roscommon comeback was on the cards but Walsh struck again for the Exiles in the 49th minute to put the issue beyond doubt.
Roscommon kept plugging away and were rewarded with another goal from a Fallon free with the last puck of the game.
Scorers for Roscommon: G Fallon 2-2 (2-1fs), M Kelly 0-5, J Coyne, K Conneely 0-2 each, S Curley (f), D Dolan (f), B Kelly 0-1 each.
Scorers for London: K Walsh 2-1, M Finn 0-6 (3fs, 2 65s), N Forde 0-4, H Vaughan, E Cooney 0-3 each, S Egan 1-0, D Maher (1f), C Quinn 0-2 each, T Dunne 0-1.
ROSCOMMON: D Dowd; S Curley, B Donnelly, D Fallon; S Melia, P Kelly, D Flaherty; G Fallon, K Conneely; J Lawlor, M Kelly, B Kelly; D Dolan, N Keenan, J Coyne.
Subs: A Moore for Melia (20), T Seale for Lawlor (41), C Egan for Dolan (48), E Mulry for B Kelly (48).
LONDON: T Williams; E Walsh, Brian Costello, G Fennelly; M Walsh, PJ Rowe; L Mackey; C Quinn, E Cooney; T Dunne, D Maher, H Vaughan; K Walsh, M Finn, S Egan.
Subs: L Hands for Fennelly (28), N Forde for Dunne (41), E Gleeson for Egan (47), G Hill for Walsh (49), T Hogan for E Walsh (51). Ref: T Ryan (Galway).