Suggestions for Nov/Dec visit to France

Most Christmas Markets start on the last weekend in November (Sat 29th November 2018), it would have to be very late in November to visit them.

Yes you could visit Strasbourg / Colmar, or London. Both are easy by train from Paris and plenty to see even if you are too early for Christmas, your choice.
If you chose London, flying back direct from London would be the easiest option.

You will not be surprised that northern France can be chilly at the start of winter, often accompanied by damp. A little less obvious, it will be getting dark increasingly early. Lots of great stuff to do indoors. Take a warm hat and rain-resistant shoes; buy a voluptuous scarf as an ideal souvenir and study how the French (male and female) drape them.

Thank you for your advice.
Sue

If you can move your trip a week or two later . . . Paris gets all decked out for Christmas, Alsace is wonderful (google "noel en alsace"), Colmar is a good base, but stay a night or two in Strasbourg. From Strasbourg take the train to Stuttgart (huge market) and day trip to Esslingen for the medieval market - one of the very best xmas markets. You'll probably have the best return flight options from Frankfurt.

Thank you to all...looking at options you have given me.

Other people have danced around it, but Christmas markets are purely a phenomenon of German culture. The ones you get nowadays in England and France are just mock-Germanic crap. The only authentic ones in France are in Alsace which traditionally has a hybrid culture between France and Germany.