"Copper and plastic pipe and fittings would never shatter, and god knows what pressure you'd need to shatter something like a rad valve. The only danger I can see is a flying endcap, so as long as a bit of common sense is used what's the problem?"
The only danger you can see is a flying endcap? Off you go, take your footpump and the best of British to you in all your endeavours.
I'm going to use some big words here, so Itandje and the other time-served half-wits had better skip this bit, lest they learn something.
First, it is not copper pipe, it is tube. Calling it pipe marks you as an amateur.
You'd wouldn't expect a copper "pipe" or fitting to shatter because, as supplied, copper is ductile. You would expect it to undergo plastic deformation, rather than shatter. Table X (sorry can't be 'rzed' to look up the EN tables) is half-hard, Table Y is soft annealed. Table Z is rarely used, it is hard drawn and is less ductile, it is more brittle; it is not suitable for bending.
The thing about copper is that it work hardens; you work it when you bend it. When it has become hard, you have to anneal it or it will break if you carry on working it. You could have a pulled bend or stressed pipe that was work hardened and liable to a brittle fracture. There remains the danger of a fitting coming loose.
I find it disturbing that a 'professional plumber' should be so ignorant of the properties of copper. I am not surprised.
You would expect plastic to be plastic. Some plastics become hardened and brittle on exposure to sunlight (notably ABS, used for compressed air, formerly used for crash helmets). PTFE is weakened by being kinked. The danger is that a pipe break or fitting coming loose will allow a flexible pipe to whip around.