This regulation clearly refers to minibuses and buses. However, if you were transporting dangerous goods you may have contravened other parts of legislation that prescribes the manner in which the goods must be transported and the duties of the operator and driver.

260.   Fire extinguishers
(1)  Every mini-bus operating for reward shall carry in a readily accessible position at least one fire extinguisher which shall be of—
(a)
the dry powder type with a capacity of at least one kilogram; or
(b)
the halogenated hydrocarbon type (BCF) with a capacity of at least one kilogram,
and which shall be in good working order.
(2)  Every bus operating for reward shall carry in a readily accessible position at least one fire extinguisher which shall be of—
(a)
the dry powder type with a capacity of at least two and a half kilograms; or
(b)
the halogenated hydrocarbon type (BCF) with a capacity of at least one kilogram,
and which shall be in good working order.