Nearer the time they will send you a travel preferences form to fill in which will contain a link to various organised all day parking at a special rate. I used their parking at the o2 last year and it was a doddle to get to the start, just followed the hundreds of cyclists heading in the same direction!
I was dropped off on the south side of the Blackwall Tunnel last year, the ride in was OK, I was pleased to be able to ride through the tunnel. I found the route on the other side poorly signed and quite chaotic, I had to put on my London commuter head now and then to make progress. There were a lot of cars snarled up in the diversions and not feeling overly cycle friendly.
Having said that, I'd happily do the same again, being a Sunday there might be on street parking around to use although you're on the edge of the slope out of the London basin there which you might prefer not to have to climb at the end of everything.
I did the riverboat back and then found there was a closed road route back to Docklands so would ride and walk through the Greenwich foot tunnel instead if given the choice.