You must mean that it relates to information that could identify private individuals, not "only applies to private individuals".And if the roofing merchant says "I'd like to tell you but I can't because of the data protection act", politely inform them that the DPA only applies to private individuals. Anyone who has a trade account clearly holds it as part of their business activities and is therefore not classed as a private individual.
The GDPR applies to business and suchlike and not private individuals in the context of who needs to carry it out.