Just done a quick trip up to London Bridge for a meeting and now returned to my desk in sunny Surrey
Used the A2 route, thru the blackwall tunnel and it took less than 20 mins from the M25 to docklands.
Rusty's route would clearly work as well, The A13 is a new road, nothing like the nightmare it was, but I think there are roadworks on the clockwise M25 by Enfield that may hamper your progress while anti clockwise there are none I can think of.
Good luck on whatever route you choose!