The gas can only burn if air gets into the pipe! I'm not sure whether the advice not use heat within 2m of a meter is due to. Either it's to prevent some sort of rare possibility of a flame going back into the meter. However it might be simply the prudent precaution to avoid using naked flames and heat near an open gas supply (albeit turned off).
The discs are made of thinnish brass sheet. The ones in my tool box are all recycled courtesy of Transco. If I attend an installation which was leaking and has been turned off there is a good chance that the Transco fitter will have inserted a blank disc into the meter outlet to a) Prove that the meter and other Tranco kit is gas-tight. b) Confirm that the leak is in the installation (home owner's property) c) Make the installation safe before leaving.
I've not looked but I expect they are in the BES catalogue the screw caps for the outlet certainly are.