Ian's got it - if you move the view north a bit you see where the A2 turns north towards the Blackwall Tunnel - the original plan as far as I can work out was for the main road to be north-south with the A2 crossing roughly at right-angles. There's rather an odd junction there that looks like it has more land than it needs.
The north-south road would have continued down through the gap towards the A20 and then swing round to the west.
Edit: Ah, I've found the original map:
http://www.cbrd.co.uk/histories/ringway2/06.jpg