I did a few days cycling around Blois and around Bordeaux as part of other holidays and I think either the Loire (route EV6) or Garonne valleys (route V80) could be the basis of great cycling holidays. So many ideas and not enough holiday time!
With some friends from a local group (is it a club? not really), I've cycled to/from Dunkirk/Dunkerque a few times on the way to/from Belgium, as discussed on other threads here. It's a long way to ride or take bike-friendly non-TGV trains from there to the rest of France, though! That's probably part of why so far we've kept visiting Belgium and the Netherlands (and Germany and Luxembourg last year) because it's so much easier to get there with bikes and there's still plenty to see.
Another tour I've read about often is the Avenue Verte from Dieppe (ferry from Newhaven) to Paris. Depending where in the UK you are, that might be easy to access.