A big group goes overnight via Portsmouth - Ouistreham; I did that my first time in 2003.
Last time a smaller group of us, led by Teethgrinder, did Newhaven - Dieppe; the main disadvantage of that trip being arriving at silly o'clock and having to ride a fair chunk in the dark before breakfast. [What am I, as a grizzled AUK Ancien complaining about that!!] We stopped overnight in Vernon, but on the way back went via Rouen, then got an afternoon sailing, catching the last train home from London. If I recall correctly though, a prettier route to St Quentin than the above.