The only realistic way to get a British qualification which will allow you to legally guide in France is to get your International Mountain Leader award + a well-recognised UK biking qualification such as Scottish Mountain Bike Leader.
To get this requires:
UK Mountain Leader – 20 logged days of hill-walking experience + 1 week training course + 20 more days + 1 week assessment
SMBL – 20 logged bike rides, 2 days TCL training, 1 day TCL assessment, 20 logged bike rides, 2 days SMBL training, 1 day SMBL assessment
IML – 20 international walking days (summer) + 20 international days (winter) + Summer training course (1 week) + Winter training course (1 week) + 10 further summer AND winter days + Summer assessment course (1 week) + winter assessment course (1 week).
If you're starting from scratch, expect it to realistically take you around 3 years and cost you something in the region of £3k.
On the other hand, if you're not fixed on France, other countries are more lenient (Spain, Austria) and even some areas of France are more tolerant (Morzine/Portes du Soliel is the hardest place to guide).