Copper pipe looks ok in very few situations. I believe USA allows TracPipe to be run underground (say to a garage or alongside a house), whereas the UK doesn't permit that sort of thing AFAIK. Probably because they would blow up a few rats & sewer network in the process.
Perfectly ok to run TracPipe underground this side of the pond. It may even be burried under concrete or embedded directly in concrete. No TracPipe fittings to be burried though.
Perhaps a "black on clear" laminated label wrapped round the pipe near to the point of exit; "Sleeved with "
When I did mine (around 2002), there were several example acceptable sleeves listed in one of the gas industry documents. That included copper and PVC pipe (and several more I've forgotten). I used copper sleeve for the 15mm and 22mm pipe (next size up), and PVC for the 28mm pipe. There was certainly no implication that the sleeve had to be regular gas pipe; AFAIK PVC isn't, but is listed as an allowable sleeve.
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