How can you know when you service a gas system who put it in? How do you know a none ACS apprentice never installed it under supervision?
Many systems over 20 years old where installed by none registered people. It is the standard of work that matter's not qualification. It is true if your paid to do a job you have to be Gas Safe registered but DIYers can still do it in their own home. If you never knew who installed the system you would never know who had done it.
Suppose a gas fitter with 30 years experience installed a system but their registration had lapsed and a three month registered gas fitter installed a system would you fail the 30 year persons system if the systems were both of the same standard?
So you would look to see if it complied with the regulations and pass it, if you never, you would have to declare every system you met ID and turn it off, because you did not know if the person who installed it was ACS or not.
Try that with a big landlord and you will probably soon find yourself out of work. The operative word is "Safe" you are employed to decide whether it is "Safe" or not, not who is qualified or not.
Why do we suppose both CORGI and GAS SAFE want to see your work even after registration and proof of qualification?
A Gas escape is not concerned with regulations but good work makes a system safer. So look at the quality not the qualification.