-
If you're prepared to train it as far as Amsterdam, you could train it down to Lyon, or even Valence, both are about 6 hours in total from St.Pancras (changes at Paris or Lille, but all included on the ticket from Eurostar), and can be done for £150-£200 return if you book a few weeks in advance and avoid peak weekend times).
From Lyon you could pick up the ViaRhona cycle route down to Valence (127km). From there you could carry on the same route to Montelimar (+74km) or Avignon (+164km) for the second day, and then on to Sète/Montpellier (+166km from Avignon) for the 3rd day, then get a train back to Lyon (about 2.5hrs from Sète, or about 1.25hrs from Avignon).
Alternatively, from Valence or Montelimar you could loop around the Ardeche, pick up some of the l'Ardechoise routes, and cycle the along the gorge to Vallon Pont d'Arc, as they did for Stage 14 of this year's Tour de France.
Or go the other way from Valence or Montelimar, direction Grenoble. Either through the Vercors mountains, which include the amazing Combe Lavale, and the Col de Rousset, or go around the Vercors, Valence/Montelimar to Gap (about 135km), and then Gap to Grenoble is about another 100km. From Grenoble there are local trains back to Valence or Lyon, both take about an hour, and bikes walk on for free.
And the weather is good for cycling in September, mid 10s in the mornings, mid 20s in the afternoons...
-
I was staying in a Gite not far from Lisieux
This is the route from the Ferry to where I was staying, it shows the way out of the Ferry terminal and inland.
https://cycle.travel/map/journey/62391These are circular routes we developed whilst we were there:
https://cycle.travel/map/journey/62398
https://cycle.travel/map/journey/62397
https://cycle.travel/map/journey/62395
https://cycle.travel/map/journey/62394
https://cycle.travel/map/journey/62393To be honest you could probably set off in any direction, it was all pretty good.
Normandy is fantastic, scenic, lovely roads, rolling terrain and easy to get to, take the night ferry from Portsmouth to Caen. Getting out of Caen is easy, take the bike path right out of the ferry port to the Pegasus bridge, turn left...
I've got a whole bunch of mapped out routes if you're interested.