Thank you everyone for all the help

We're going to look seriously into maps in the next couple of days, the IGN Yellow (green in the past) the Michelin and whatever else IGN suggests. I think unless there's a solid cycle path which is tarmac we may want to stay on-road because this year we had many issues trying to find these cycle paths, but if the road is so good we may as well stick to them.
The weight of evidence is with our meths trangia, so I'm going for that (gas was convenient but difficult at times this year).
Transport is still an issue. Will Eurostar hike their fares massively during holiday season? We'll probably be leaving shortly after July 22nd and returning up to three weeks later. Eurostar seems best at the moment because it takes us directly to our start point at quite a good price and we're pretty short on time - although if they do up the price Newhaven > Dieppe seems like a good second option, but we would have to take the train into Paris since that's already some significant cycling.
Thanks for the tip about the Alps, we'll almost definitely be going to Provence, so if anyone has any suggested routes both for that or any other leg of the trip, it would be very much appreciated.
Is there a specific route that we should take? Going through the Parc National des Ecrins seems like a good idea.
Thanks again.