If you want the Eltham to Dover route, there here is our route over two days last year
This was with a group of inexperienced cyclists, so all main roads were removed.
Day one Woolwich to near Sittingborne
Day two Sittingborne to Deal
Day three was the 10 miles Deal to Dover (about 90 mins) for a 10am Ferry
https://cycle.travel/map/journey/60109
https://cycle.travel/map/journey/60113
https://cycle.travel/map/journey/60115
Remember you lose an hour crossing to France, with the benefit of hindsight the changes we would have made were:
Day One: We would have got on the train at Eltham and started from beyond the M25, either at Dartford, Rochester or better still, Sittingbourne.
Day Two, Sittingbourne/Deal was great
Day Three: Deal to Dover - nice but a bit of a struggle
Dover/Calais - Allow 4-5 hrs from port entry to port exit (remembering you loose an hour. You may be better off going to Dunkirk
(Although for Paris, Dieppe is way better)
Next time we would have cycled 10-15 miles out of Calais/Dunkirk and then called it a day.
(We did Deal to Dover, Ferry and then Dunkirk to Ypres all in one day. 25-30 miles to far for the inexperienced cyclists.)