The fact that they have show use only written all over them should answer your question, but in law -:
Danger of Injury
Section 40A Road Traffic Act 1988 - level 4/5 fine 3 points
A person is guilty of an offence if he uses, causes or permits another to use a motor vehicle or trailer on a road when :-
• the condition of the motor vehicle or trailer, or of its accessories or equipment, or
• the purpose for which it is used, or
• the number of passengers carried by it, or the manner in which they are carried, or
• 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.