In reply to Wildtraks:
Technically, I believe that, in the UK, you do not need any formal qualifications to work professionally in the hills with adults but finding an insurer may be an issue. The problem comes, if/when you have an accident and there is a court case. The court will then as a qualified instructor, normally a guide to review the case and judge if you were sufficiently experienced to be doing the activity and also if your actions were negligent and contributed to the accident. Now, having an ML/MIA does not mean that you cannot be negligent but you are likely to have a better defense on the experience issue. As has been mentioned many times on this forum, there are many really experienced people without qualifications and also many relatively inexperienced people with qualifications but I have generally taken the provision of a qualification as the start of the professional journey in the mountains and I am sure that a potential client will realise that having a certificate shows that you have received training and assessment and reached a certain standard.
I obtained my quals (SPA, ML & IML) after having 40 years experience in the mountains and have never regretted doing it.