There are a number of problems in my view keeping your UK mobile number (not least of which the ultimate effects of Brexit).
Firstly French people will tend not to phone a UK mobile because it will cost them money.
Secondly, on the same basis, if you phone them it will cost them money.
Much easier to get a French SIM.
If you have a mobile phone which can tale 2 SIMS then maybe keep the UK one for phoning the UK but as an alternative there is always the possibility of using Whatsapp for friends and family in the UK which will work either on your phone or WiFi in your rental cottage.
This thread will give you further ideas.