The reason you can't take an LPG car on the tunnel, but you can take a caravan with LPG bottles on is quite simple... or so I have been told ......

When the chunnel was simply a concept all sorts of "risk assessments" had to be compiled covering differing fuels/volumes/types of vehicle/number of passengers etc etc etc
Quite simply, no "risk assessment" for LPG powered vehicles was carried out, as they simply were not considered - probably due to very low numbers of such at the time - but caravans with LPG cylinders were considered so a "risk assessment" was done.
The licence to operate the chunnel is based on adherence to the factors and procedures laid out in those assessments. So no "risk assessment" equals no licence to carry.
The obvious question is "why not simply do a risk assessment now" .. the answer is cost, both the cost of actually doing the assessment and also the cost of then implementing any additional factors it identifies....... for the number of LPG powered vehicles that might wish to use the chunnel, the costs far outweigh the commercial viability.
That's my understanding of the situation anyway ...
