You've linked to an HSE page which is dedicated to protecting employers in the workplace. Houses are not places of work.
True, yet it states that:
"Who is competent to work on gas fittings?
Domestic properties, schools etc
In domestic properties and workplaces such as shops, restaurants, schools and hospitals, this must be carried out by someone on the Gas Safe Register who is qualified to work on gas appliances.
It is illegal for an unregistered person to carry out work on any domestic gas appliance. You can check this by contacting the
Gas Safe Register 
online or by calling them on
0800 408 5500.
All those who are registered carry a Gas Safe ID card, which shows the type of work they are qualified to do and whether their qualifications are up to date."
If we look at the regs it states a competent person, and the regs also seem to indicate that the HSE has some sort of authority of this area as they are the only body who can give exemption certificates from following the regs.
It would be common sense that a professional department such as the HSE can determine competence (which appears to what is being argued in this thread), and they state that competence is someone who has sufficient training
and experience/ knowledge. As well as what is quoted above. I mean, reading a manual and thinking you know what you're doing, in regards to something potentially life threatening to yourself and others within your household and vicinity, is arguably not competence and arrogantly dangerous.