Heliotrope Smith coughed up some electrons that declared:

It must be capable of containing or distributing gas.

When a pipe goes across a cavity the sleeve is meant
to prevent any gas escaping into that cavity and building
up into a dangerous situation.

One end of the sleeve will be left open to carry away any
leaks (usually to the out side air.

Although it's more of a backup, rather than normal mode of operation.

I can't see a little bit of gas leakage (which is itself unlikely in a short
length of continuous pipe) permeating through, say, a bit of thick 40mm
plastic waste pipe into the cavity before rolling on out the other end to
free air.

What I'm getting at, is whilst these rules are written with the best of
intentions, aren't we worrying a bit too much?

I can present counterarguments why sleeving in copper or iron is prone to
failu

a) The copper sleeve will be at as much risk of chemical attack by the wall
as the original pipe;

b) Copper and iron is a bad combination as there is a risk of dissimilar
metal corrosion with the copper being sacrificial.

Not to mention possible friction wear as Andrew mentioned.

I'm not questioning your excellent knowledge of the rule book, but stepping
back and using common sense and a bit of basic science, it seems to me that
a bit of PVC or PET pipe, gas approved or not, is actually the best thing
to use as a sleeve, or if using an iron sleeve, then denso round the copper
would be a good idea to avoid corrosion and friction.

Cheers

Tim