The regulations (law) state that only a competent person can work on un-vented HW systems and their associated safety systems. In the HSE eyes, competence can only be asserted by individuals passing and obtaining the relevant qualifications, very similar to the rules governing gas work. Therefore if anything did happen and individuals were injured, as you don't have the qualifications, you would be criminally responsible.
I and the others are not trying to be cheeky or insult your intelligence or even having a go at your proficiency, ability or the right to learn and beter yourself, that's the rules & regulations plain and simple and therefore the law I'm afraid and what everyone else is saying!