Gas plumbing comes in two parts: the incoming supply up to the meter and then the pipework and fittings within the house. Normally the procedure is that you purchase a meter box from the gas supply company and fit it externally on the house in a position where the meter can be easily read. The gas supply company then run a supply from the nearest main and provide a meter and connect to it. They charge for this depending usually on the length of the run. It is then your responsibility to do the internal pipework.
There are various rules about how the incoming pipe enters the building to avoid explosions in the event of gas leaking at or near the point of entry. There are also rules about not running through one flat to reach another. The pipework is normally in copper or iron. See the IGEM guidelines on installation at Gas installations in timber framed and light steel framed buildings
These points should be covered in the drawings that have had Building Regulations approval.
Because gas is potentially so dangerous it is important that the installer / plumber is competent. There are some quite interesting forum postings on what constitutes competence here.
See the Gas Safe Register
