Totally disagree, and your analogies are wrong! Aspirin are not a controlled substance, but something like clinical grade heroin is. Can anyone walk into a chemists and buy that heroin? No. The reason is that it needs to be prescribed by a competent person.
If gas works can only be done by a competent person, and there are regulations in place to define what a competent person is, then there's no reason a non-competent person should be able to buy those things. It's not just about protecting people from themselves (the DIY'er who might think they can install their own gas boiler) but also unsuspecting people from dodgy cowboys.
The problem I see is that the definition of competent is open to interpretation in court and isn't as simple as 'is on the gas safe register'. If it was that simple then anyone not on the register shouldn't have a need to buy those things.