Propane bottle regulators

We are installing a propane gas water heater, a multipoint type boiler.

I know about low and high level ventilation to provide crossflow of air etc., but am puzzled by the advice I have been given (by a heating engineer) that none of the gas regulators and safety switches that I can buy in the Bricos or builder's merchants are acceptable for insurance purposes in case of an accident.

They all run at 37mb and being for propane are suitable for outdoor, albeit covered, use.

The inox flexis are all guaranteed for 5yrs.

We plan to install a soldered copper supply through the building wall to connect to the appliance on the inside.

Can anyone tell me what I'm missing please?

Thanks in advance