Currently researching this, would appreciate thoughts from those that know.
Who's done it, which one, why, does it matter?
MIAS: http://www.mias.org.uk I know it's used but the quality of their website (covered in ads) doesn't indicate this is more than a one man band.
CTC: I never see the CTC as an off road organisation really, I know they do, but more "rough stuff fellowship" type thing.
British Cycling: http://www.britishcycling.org.uk/mtbleadership probably the best known brand amongst potential guiding customers, perhaps it has a chance of getting recognised abroad too. On the other hand BC's heart is really in competition/coaching not leisure/adventure cycling, and I have a concern about getting drowned in politics and paperwork.
The use case is group leadership not skills training.