If a gas supply was to be cut off, then there would be a legal duty upon the operator to ensure that any gas meter and gas appliances were appropriately labelled. (They need to be clearly marked, in order to ensure that they're not left turned on at a time when the supply is subsequently restored. If they were, the property could fill with gas, with a consequent risk of explosion). So representatives of the gas company could seek a warrant, under the provisions of the Rights of Entry (Gas and Electricity Boards) Act 1954, to enter the property.
However it has to be remembered that trespass is not a criminal offence anyway (other than in certain specific circumstances, which aren't relevant here). If you leave you home not properly secured (e.g. by leaving a door or window open or by leaving a spare key where it can be found), anyone can legally walk into your home if they've not got any intent to harm you and/or steal or damage anything inside. That equally applies to anyone who owns a set of skeleton keys, as may have been the case here.