Please note that only Gas Safe registered people can work on gas appliances. All gas appliances need to be registered with Gas Safe register when they are changed or installed. This includes cookers, gas hobs and boilers.
There are strict rules about the amount of gas required for every appliance. If there is not enough gas available for the requirements of the appliance then the appliance could be dangerous.
It is usually possible to do a rough calculation to establish whether there will be enough gas for the appliances that will be connected. However, this can never be definitive especially when pipes are concealed etc.
Unfortunately it is impossible to test that the correct amount gas needed for the appliance to burn correctly without having the appliance connected.
If when the final commissioning checks are carried out and there is not enough gas, then the appliance fails. It will then require that the pipework be adapted or re-run so that enough gas is delivered to all the appliances working off that meter.
Re-running the gas piping to an existing appliance can lead to considerable disruption and cost.