In order for it to be illegal work, it has to be established that the person who did the work, should not have.
That would apply anywhere in the World.
So consider this,
The builder is questioned about who did the work related to the boiler.
He says he got someone in who said they were Gassafe registered.
The Gassafe guy disconnected the gas, the builder removed the boiler (hence his DNA and fingerprints are all over it)
The builder helped the Gassafe guy fix the boiler on the wall and the Gassafe guy connected the water and gas and commissioned the boiler and was wearing gloves.
The builder paid the Gassafe guy cash, with the promise that he would send on paperwork for the boiler install.
He didn't and the builder has tried but has been unable to contact him.
What's the builder guilty of?
What's the non existant Gassafe guy or gal guilty of.
Or the builder could just as easilly deny ever having touched the boiler, (DNA and fingerprints might disprove that unless he was wearing gloves.
You'll need evidence the builder was asked to move the boiler.
Evidence he personally did move it and leave it switched on and working, whether safely or not.
Evidence that he was paid for it having invoiced for the work might go against him, but doesn't prove he actually did the work.
If the boiler is found to be unsafe or has injured someone then the incident is more likely to be thoroughly investigated, but the outcome could still be the same as above.