If you have an iPhone SE 2nd Gen or 3rd Gen, eSIM is supported.
When you are in France, you must find a Carrier that offers eSIM to their customers. A "pre-paid" option will probably be the most optimal if it is only to be used when you are in France, and if it is only to be able to buy things with your French number.
eSIM is usually installed just by scanning a QR code, then the SIM card is ready for use and you can make calls and send messages with that phone number. You UK number will be active at the same time, and of course you just keep your normal SIM card with your own number in your phone.
When you are back home in the UK, I would recommend deactivating the eSIM card, or finding out what it costs with the Carrier to roam in another country.
You can find Carriers that support eSIM here:
Find wireless carriers and worldwide service providers that offer eSIM service - Apple Support