OK, I'll take the other side of this argument.
Bearing in mind our business model, you might expect me to be strongly in favour of this concept, but after thinking about it for a couple of decades, I have come to the conclusion that its a bad idea.
There are two main reasons - one of principle and one of practicatily.
Firstly, we live in a free country. The default position ought to be that anything is allowed, and that if things that are banned or restricted we have these controls because there is clear and demonstrable evidence that by limiting an individual freedom, there is a greater benefit to society as a whole.
We have several levels or degrees of restriction.
1) Very dangerous items, like firearms, explosives, some drugs and a handful of other items are controlled at point of ownership with laws that are generally enforced pretty thoroughly and are only allowed to be held by people who can demonstrate a clear need for them, and has the relevant protections and controls to stop them from falling into the wrong hands.
2) Another category, including motor vehicles and gas appliances, are free to own, but are controlled by licencing the skill of operators at point of use (ie driving licence, gas safe quals) and is usually enforced by exception - we wait until there is a problem before we deal with enforcement. Otherwise you would have to prove you had a licence every time you got in your car, or bought petrol.
3) There is a final group - including alcohol and cigarettes, which we acknowledge as a society are dangerous, but our only attempt to control them is to prohibit the sale to under 18s. This is barely enforced at all.
What is being suggested is that gas spares and appliances are so dangerous that they should be moved from category two up to category one (along with firearms and class A drugs). I simply don't see the benefit to society that this would bring. Although every death is tragic, there simply are not enough deaths due to preventable DIY gas work to justify such a draconian change. If you really think that every life is so precious than any restriction is worth it to save just one - then you should be arguing for a 20mph speed limit on all roads, or a total ban on alcohol and tobacco!
My second point is a purely practical one. Banning the sale of gas spares does not mean that every job will be done by a competent engineer. In some cases it simply makes it more likely that a DIYer will use an inappropriate product. Can't buy a gas cooker hose? Don't worry, a length of garden hose and two jubilee clips will do! Can't buy a gas-rated valve? Never mind, use one designed for water.
Just as prohibition created more problems than it solved, I think that this would actually lead to more DIY disasters than it would prevent.
Let me offer an alternative solution. Spend less than 10th of what implementing such controls would cost on:
a) a really good and sustained public information campaign about the dangers of DIY gas work
b) prosecuting substantially more cases of shoddy and dangerous workmanship
I'm very sorry to say that if we could track back the origins of all the "hall of shame" jobs, many would not be DIY. They would be done by "qualified" blokes.