Hi Cendrelune
I’m not sure you’ll be able to watch for free, except via some rather dodgy web-sites which are full of adverts to pay for the service, and which are often supposed to be liable to introduce viruses to your computer.
You might want to visit:
https://www.uktv-online.com/uk-tv/watch-uk-tv-online-in-france/ which gives you the various options, which include paying for services which host your internet connection in the UK, which convinces the TV streamers that you are watching legally from the UK. One such service, which I haven’t personally used, is tvmucho.com which I believe allows you to subscribe for about 6 euros a month, presumably allowing you to terminate the agreement after just a month, which might allow you watch both Cheltenham and the Grand National for a single payment?
Good luck!
Oscar