France
For France, I've made my suggested strolls as a set of downloadable PDFs along with interactive Google maps that you can use to go for a wander.
Getting there (from the UK) and around
Train & Ferry
For details on getting to and around France by train, as always, my first stop is The Man in Seat61: https://www.seat61.com/France.htm
Eurostar to Paris (or Lille) has become the iconic way to get there.
Check out Rome2Rio for the more obscure options.
You can also find out journey and ticket information from the national rail operator SNCF:
Coach
A plan B for when Eurostar is full or too expensive is to take the coach, which take around 9 hours to get to Paris from London.
Tourist Info
French tourist information have put together some ideas on travelling to and around France sustainably here:
https://www.france.fr/en/campaign/travelling-sustainably-in-france
Guide books/sites
Rough Guide: https://www.roughguides.com/france
Lonely Planet : https://www.lonelyplanet.com/france
Trip Advisor: https://www.tripadvisor.co.uk/Tourism-g187070-France-Vacations.html
Accommodation
If getting your place through booking.com, there's a Travel Sustainable option.