I am asked quite often how to become qualified to take groups out on the hills. Sometimes people just want to confirm a higher level of personal skill.
OK, ML. Actually ML (summer). The main website for Mountain Training is here: http://www.mountain-training.org/walking/skills-and-awards/mountain-leader
In summary, you think about what kind of work you want to do and what qualification is necessary. MLs is widely respected and pretty much covers everything in the hills in GB except rock climbing and winter mountains. Obviously though not canoeing, mountain biking and the like.
You can do other, lesser, hill skills qualifications but no cheaper and not worth missing out on the full deal.
You need to have experience in at least three major UK mountain areas. These would be The Lakes, Scotland and Wales although The Peak, Dartmoor and Northern Ireland count as well.
You register on the scheme with Mountain Training and fill in a log book. This would show your experience in mountains going back as long as you can remember. The minimum number of Quality Mountain Days (QMDs) to attend a training course would be 20.