Drat - just lost a long post before submitting !!!!
OK, summary...
Risk assessment is only legal requirement. However, proving competence in a court of law is easiest with an NGB qualification.
Disclaimers aren't worth anything if the person signing isn't judged capable of making the decision. That includes experience of activity, not just mental state. So someone with no experience in the mountains couldn't sign away responsibility for a mountain walk.
That brings us to insurance, which is nigh on impossible to get at an affordable price without a recognised qualification.
Experience is undoubtedly key, but normal walking rarely equips people with the full set of skills required to safely lead novices. That is where formal training comes in.
It is easier to market an 'experienced and qualified leader' over an 'experienced leader'. You can include branding from the Mountain Training Association if you are a member. You can advertise in places that only allow qualified leaders to place ads.
Obvious quals from Mountain Training:
Lowland Leader - eg. Quantocks
Hill and Moorland Leader - eg. Dartmoor or Brecon Beacons
Mountain Leader (summer) - eg. Snowdonia or Cairngorms
Some organisations have constructed alternatives. The Rambing Association, I understand, is now doing an in-house leadership course that incorporates a National Navigation Award Scheme award. Doint that as an individual, however, is unlikely to be easy.
I should point out that I have been an ML for many, many years, and am also a provider for Gold NNAS courses. As such, I am firmly in the NGB mould. My response is aimed at commercial walk leading; I'm not suggesting informal walk leaders be obliged to go through a formal process. However, I would still recommend some sort of skills course for anyone getting into mountain walking who doesn't have experienced people to draw down knowledge from.