Hi PM,
If you are taking the M25, I might be tempted to go north around it and come along the A13, I live right by the Dartford crossing and have both options of A2 and A13 when going to Excel. The A13 is the quickest and most direct route as far as I am concerned as using the A2 you end up heading out of town again once you've gone through the Blackwall tunnel, you can get held up in road works at the tunnel particularly at night and at the weekends.
Rusty