An electrician can not open a boiler if its case forms part of the combustion seals. Ive lost count the number of cases ive seen not fitted back on correctly by electricians, maybe this needs addressing by the competent persons scheme's for sparkys. extract fans and open flues should be addressed aswell.
No sensible gas safe engineer would knowingly connect the gas and sign off a boiler fitted my a non competent person. Thoses that do are complete idiots and hopefully will get found out and removed from the gas safe register and more.
If your signing to say you have installed said appliance you are responsible for the screws holding the appliance on the
wall and all other aspects of the installation. The only safe way to do this would be to uninstall the appliance and visually check the entire gas installation, i would charge more to do this than install the boiler in the first place.
All because you have seen "plumbers" fit boilers and an engineer sign them off does not make it correct, those companies that do this are breaking the law and doing the industry a dis-service.