Hmm!
Some old chestnuts bubbling away here!
Gas fitters where until about the 1970's regarded as part of Plumbing and still are. But they where thought of as semi skilled by some, simply because they could not usually do the full range of a Plumbers job. Whereas a Plumber was expected to be able to do the full range of a gas fitters job.
The reason they seem to be two separate trades now, is because of the old British Gas company. They moved into the domestic central heating market and demanded that their gas fitters start fitting central heating. It was a marketing ploy not a skills move that separated the two. On one site we where pretty "pally" with BG guys and some of the lads would go down to a neighbouring site in their dinner hour and help the BG gas fitters by telling them how to do the central heating. I must admit I suppose a few pints where part of the bargain.
Anybody looking into the history of BG will probably find the reasons why the domestic heating industry is like it is now. They virtually dominated the whole domestic heating market and dictated to it.
I suppose you could say its a bit like the ceiling fixer and the joiner. The joiner installed suspended ceilings but it became a market on its own so some joinery company's started to specialise in ceiling fixing. You could go on, a brickie and a plumber both learn drainage but its usually specialist drain layers who now do most of the work.
I have worked on site's where the company I was working for has been doing the plumbing and another the central heating, gone to another site where, we where doing the central heating and the other company the plumbing.
Its the same with roof work and flashings.
However the Plumber has to learn, drainage, central heating, gas fitting and possible basic electrical. In Scotland Plumbers wired the houses out as they protested when electricity first came out that the electricians where pinching their gas installation work.
Incidentally the original central heating engineers went to a property usually large properties, measured up and then went back to their factory and made the rads and pipes usually by casting them in iron.
That is why they are called engineers and not pipe fitters.
The Plumber also did the fitting of the rads and pipes, but not the engineering.
Now as I explained the gas fitter/plumber/central heating engineer all do it.
And we have not even mentioned glazing yet!!!
Lets be honest, its all about earning a crust and what you think you can do to earn one and interest of course.
All this inter trade war and perhaps qualification snobbery is just silly.
If you go on a course all you learn is what somebody else has found out. The British Standards change nearly every year because they find new things out all the time. The ACS is done every five years because technology changes. So I don't suppose anybody can say they know it all.