With water some ‘plumbers’ are certified by Water companies to do the mains laying, testing and connection so may be similar for gas.
Alternatively look for a self lay company
Thanks, that search term helps.
Edit: actually it looks like self lay is for much larger developments, but that did help me find an approved contractors page on the Welsh Water site, so will make some calls tomorrow.
@Polygon started
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With water some ‘plumbers’ are certified by Water companies to do the mains laying, testing and connection so may be similar for gas.
Alternatively look for a self lay company