Thank you for all your comments. I have decided to go London - Paris Gare du Nord, taxi across Paris and then Gare de Lyon to Nice. I'm not the quickest walker, so am allowing 1.5 hours to get off the Eurostar, grab a taxi, and board the TGV. Is this sufficient time? Are there any hints for minimising any delays? E.g., is it best to get seats at the front of the Eurostar so that I can get out of Gare du Nord quickly? Are there any passport checks leaving Gare du Nord, notwithstanding any changes which may occur due to Brexit.
I am aware that over here, if there are any delays on your journey such that you miss your connection, you can catch the next available serrvice. Does such an arrangement exist in France if I am delayed on the Eurostar? Does it matter if I break the train journey by taking a taxi?
1. You have allowed ample time so:
2. SNCF is part of Railteam which operates such a policy. It is best to get the delay confirmed by Eurostar staff on board. How you cross Paris makes no difference.
3. However, I don't know whether the policy would extend to their Ouigo low-cost services (there's one at 17.12 from Gare de Lyon); I suspect you would want to avoid the risk anyway. Maybe someone else on here knows?