Hmmm, yes it might help break the catch-22 that you can’t buy a mobile phone or SIM in France unless you already have a French mobile phone.
It is subtly excluding, although I presume going to a physical phone shop in France allows purchase of a phone or SIM de novo.
No good for longer term use though as I also want data - either for the “DIY box” or my phone (as a backup).
Oh, and a French SIM might actually be useful in the UK as it will roam to any operator - handy if your regular service provider has no signal.