So there goes the argument that carrying barrels instead of properly fitting a tank means that you are just carrying 'cargo' and don't have to insure it differently..
The unsafe load of barrels etc could invalidate any insurance anyway!
The penalty for carrying an unsafe load can range from a verbal warning to a £60 fixed penalty notice. Drivers can also be issued with an immediate prohibition notice.
http://www.dft.gov.uk/pgr/roads/vehicles/vssafety/safetyloadsonvehicles.pdfpage 111 on the above PDF link states this:
Regulation 40A of the Road Traffic Act1988
introduced by the Road Traffic Act 1991
40 A. A Person is guilty of an offence if he uses, or causes or permits another to use, a motor vehicle
or trailer on a road when:
(a) the condition of the motor vehicle or trailer, or of its accessories or equipment, or
(b) the purpose for which it is used, or
(c) the number of passengers carried by it, or the manner in which they are carried, or
(d) the weight, position or distribution of its load, or the manner in which it is secured,
is such that the use of the motor vehicle or trailer involves a danger of injury to any person.Appendix A
111