If an installer leaves an uncommissioned appliance connected to the gas (even if it's turned off at the boiler) he is required to leave a notice to that effect. If he hasn't left the notice, he's committed an offence under the Gas Safety (Installation and Use) Regulations, and should be reported to the HSE under the RIDDOR regulations.
Riddor looks like its only for accidents, but it's also appropriate for situations which are left unsafe ("Immediately Dangerous") though incompetence or bad workmanship.
"When gas operatives carry out new installation work in customer’s premises, they are required to
ensure that the appliance / installation is installed and fully commissioned in accordance with the
GSIUR and manufacturer’s instructions. If this cannot be achieved, the appliance / installation must
not be left connected to the gas supply. If an appliance cannot be fully commissioned, the gas
supply must be disconnected from the appliance and sealed with an appropriate fitting. It should be
labelled to the effect that it must not be used until full and proper commissioning tests have been
carried out."
Obviously you'd report him to the Gas Safe Register as well.
Commissioning one of these boilers, even for an empty (water) system, if it's all connected up, would take under half an hour. So he's being a lazy git.
He might have to come back later to balance radiators, but at least the heating & hot water would be ON!