Heh! 😉
No, seriously, it might actually be easier for you to park up nearby, and just pop on the DLR/tube to North Greenwich. Where I am, is just the other side of the Blackwall Tunnel. I probbly only live a mile from the O2.
That way, you could avoid the chaos trying to get away from the Dome, and be straight onto the A12, up to the M11, M25, M4, Bosh! Home. 2 1/2 hours?
I often hear of proper jams on the Blackwall Tunnel southern approach, following concerts. You might be lucky, though, but check things out first, re parking etc.
Coming in on the A2 would avoid the BW Tunnel entirely. It takes me and Ollie about 1 1/2 hours to get from mine to Swinley, so about the same to the M4 junction, I’d imagine.