This doesn't seem to make sense regarding pressure relief valve pipework, since there won't be any pressure in the pipework before the PRV lets go.
on another note I found this article that mentions the building regs:
(http://www.johnguest.com/linkpages/SPFinstADV.html)
"In accordance with current U.K. Building Regulations (Part G), discharge pipes from temperature and/or pressure relief valves must be run in metal pipework."