I feel I am obliged to comment on what are dangerous activities. Having seen the devastation that gas leaks can cause, and being aware of some of the legislation surrounding safety issues, I would be negligent if I did not point out what is a dangerous practice.
RogerL suggests that a private individual is not obliged to comply with the safety aspects of carrying gas bottles. Whilst many people do ignore the regulations that does not make it right or safe, and as it is safety related, any prosecution would be under Health and Safety legislation and is subject to the criminal code, not the civil code.
It is simply not sensible to carry gas bottles in an enclosed car.