The Dartford crossing wouldn't be an option.
There are limits on numbers, types of bikes and the times of the crossing.
You have to contact them in advance if there is more than 3 in the group, or if the bike is non standard (Tandem etc). They class non standard as anything that won't go on a regular car bike rack.
It is also only operational between
3am to 9am
10:30am to 2pm
3pm to 9pm
10:30pm to 2am
Owen