Trip to France and London

Dijon, about an hour by train from Paris, might suit you. We stayed at the Kyriad hotel not far from the RR station a few years ago. From there it is a pleasant walk to the centre of town.

I'm not very well travelled in the area directly between Paris and London, but one suggestion for somewhere that won't have a very long trip back to London would be Strasbourg. It's an attractive city and due to the German cultural influence has a Christmas market. You can get to London from there easily by train as the train from Strasbourg arrives in Paris at Gare de l'Est, which is only a short walk from the Eurostar at Gare du Nord.

Another possiblity might be visiting somewhere in Belgium like Brussels, Bruges, or Ghent (note that Bruges or Ghent are not French-speaking, but practically everyone you meet there will speak either English or French, probably preferring English due to the regional rivalry with French-speaking Belgians).

The biggest warning is NOT to plan to travel into London on Christmas Day or Boxing Day, as transport will be non-existent (on Christmas Day) or very limited (on Boxing Day).

Consider Lyon, one of France's major cities, just 2 hours from Paris by train.

Dijon, Strasbourg, and Lyon are good choices (one only though!) I'd suggest Normandy (e.g. Bayeux) but it may have the worst winter weather in France, especially on the D Day beaches. I think you want a big, or fairly big, city within a short train ride of Paris. Another option might be Brussels, where you can get the Eurostar straight to London, but I haven't been there.

Thank you for your replies! They are really helpful!

I also wanted to know if it will be ok if I travel to London on the 1st of January. Will there be public transport available?

New Year's Day usually has a Sunday service. It's possible that there may be some lines out of action due to engineering work, but it's too far away to be certain.

+1 for Lyon, we loved it there. Unique sights, great public transportation, incredible food.

If you plan to travel from Paris to London by Eurostar train, then buy tickets soon to lock in one of their better fares, which have limited seats available.