Since 1st July 2012 it has been illegal to drive in France without an NF approved breathalyser in your vehicle. The breathalyser must be NF approved and if it is a single use product, unused and within its expiry date. It is recommended you always carry two disposable breathalyser, so if you use one, you always have an unused one ready to produce to the Gendarmerie. You should purchase the breathalyser before you leave the UK.
We recommend the AlcoSense NF Breathalyser which is actually used by the Gendarmerie for road side screening. They are easily available from Halfords or direct from www.alcosense.co.uk for around £5 for a twin pack. AlcoSense are one of only a handful of manufacturers officially NF certified by the LNE Laboratories in Paris.