Claim the train fare from BA after you travel. They're required to pay extra transport costs between the airports you bought a ticket between and the ones they transported you between for rerouting due to a cancellation.

BA aren't very good at paying that for you at the time, you tend to have to claim it back.

See https://www.flyertalk.com/forum/brit...61-2004-a.html for more info. That thread also covers the circumstances under which you can claim compensation, if the aircraft had a tech fault you can claim delay compensation. If it was the French ATC, bad weather, etc, you can't.