The route I took involved getting the ferry from Gravesend to Tilbury. It runs every 30 mins apart from an hour (or maybe hour and half) gap at around 3 ish. So fairly regular, but not regular enough for me to avoid stiffening up! It's £3 one-way which I thought was a bit steep considering it only takes a few minutes to get across.
This is all off the top of my head, but I think that's the most Easterly point you can cross the Thames. Next one to the West would be Dartford. Obviously you cant go through the tunnel or over the bridge but apparently there's a land rover that does shuttle runs for cyclists, although I couldn't find exactly where you had to be, how often it ran etc. Plus the fact the roads around there are much busier. Further West, the next one I know for sure is Tower Bridge so that's central London which obviously I wanted to avoid. I imagine there might be another ferry between Tower Bridge and Dartford, but that's still a bit too central London for me.
As for the route, I really enjoyed the Chichester > Gravesend side of things. There's a couple of fairly busy A-roads involved, but no more than 5 miles worth in total. I'm sure you can avoid them altogether if you didn't mind doing a few extra miles. It's also the flattest route I could make as well (wuss!). Again I'm sure you could circumnavigate some hills but you'd increase the mileage.
I don't know if it was because I was pretty shot to pieces by this point and therefore fed up, but Tilbury to Chelmsford wasn't as good. Busier roads, and had to go through the centre of a couple of towns. I'm sure I could've improved the route with a bit more research, but again not without upping the mileage considerably.