You could expand the ride into a two day ride. This would enable you to move further away from London, use more quiet, scenic and easier to follow roads. Essex, Herfordshire and Bucknghamshire have some very nice lanes for cycling. Maybe follow the Pilgrims Way in Kent?
The Gravesend Tilbury ferry is much more enjoyable than the Dartford Crossing. A much better ride o and from the ferry too, allthough the Gravesend side seems the worst, from my memory of my experience. But I don't know Kent very well. If there are too many of you for one ferry crossing, you could always split. One lot go to a pub for food, while the others cross the Thames, then stop at a pub in Gravesend for food, as they wait for the other lot to re-join them when they get off the next ferry.