ACOPS and ACS where and are like a driving test to see how much you know about gas and its installation.
Basically they give you questions to answer and usually a job to describe in all related gas terms to check whether you know the regs for the job.
Err! They don't give you a straight forward job usually but one that involves a grey area.
Then they take you into a room and tell you to fit a boiler, test a fire, size a vent, size a flue, work out gas rates and things like that in a real world type of situation.
They aren't lessons that teach you about gas. If you want lessons about gas you pay extra. And the examiner is not supposed to speak to you as you do it.
In other words they are a sort of driving test for gas installers.
Gas Safe does the same thing, only it goes around and looks at actual jobs you have done. And they don't tell you they are coming either. Which is a good idea. So better get and keep all your jobs up to standard boys.