Relevant Law: Excise duties (personal reliefs) (fuel and lubricants imported in vehicles) order 1989
This allows individuals to bring fuel into the UK in their private vehicle without payment of excise duty, provided the fuel is contained within the standard tanks of the vehicle or in a portable tank not holding more than 10 litres for that vehicles use. (Regulation 4).
This order does not apply to commercial vehicles. The regulations stipulate the type of fuel tanks allowed.