why do cars with LPG kits, still need to use Unleaded Petrol to run?

:)

Are you sure you don't have alternative fuel conversions confused?

A diesel car modified to run on waste vegetable oil requires regular diesel fuel just to warm the system up and get the vegetable oil to the point where it will flow and burn without leaving too much residue so from that perspective there are alternative fuel conversions that require an initial start on conventional fuels but spark ignition engines such as gasoline has always been able to use gases such as natural gas, wood gas, and propane with just minor changes such as in flow rates without starting on conventional fuels.

It may be that perhaps the gasoline engine conversion was a simple pipe into the carburetor hence it was safer to first get a downdraft airflow going by starting the engine on gasoline before turning the valve on with the propane. Just turning the valve on the propane before the engine has turned over is more likely to disperse it throughout the engine compartment and potentially be ignited by the sparks from the starter motor. Of course this problem could be solved with a blower, computer valve control with airflow sensors or a larger starter motor so that the motor could be turned to establish an airflow before the gas valve is opened.

The forklift truck as a purposed built Propane vehicle probably had an interlock that prevented the valve from being opened if there was no airflow into the engine and probably had a larger starter motor to establish a better airflow when starting.

As to LPG needing to be warmed up, that's just the nature of any compressed gases and the heat would simply be absorbed from the environment, if you've ever put freon or freon replacement (R134a in the US) into an air-conditioning system you will have noticed how the can of gas gets cooler as you feed it into the system, that's the same way LPG will get the heat for the expansion of the gas, a more mundane example would be how an aerosol can gets cooler as you use it in a sustained fashion, once a common experience when liberal use of hairspray was popular. LPG only has a liquid component because at the pressures that it's stored at, some liquefies and the rest stays at a constant vapour pressure at least till all the liquid boils off, this gets around the usual problem of a linearly dropping flow rate as the compressed gas is used as fuel. The amount of heat being absorbed would be proportional to the amount of gas being let out of the tanks so it's very unlikely that an additional heat source would be needed with an LPG system but the cooling effect would've been noticeable hence a myth that heat is required could easily have developed. Essentially, ambient temperatures, even in extreme arctic weather is plenty warm enough to expand propane from it's liquid vapour pressure to atmospheric pressure, it just has to be above -44 degrees Fahrenheit. If freezing was a concern, simple aluminum fins on the can and some airflow across those fins would be enough.

either you've been misinformed or its a problem peculiar to the U.K.

the LPG powered vehicles in the U.S. have only a LPG tank as a fuel source.

for example a local theme park has passenger tugs with 1970's era chrysler slant six's that run summer & winter only on LPG.

and many local large buildings have standard caterpillar diesels hooked up to the buildings natural gas line powering their air conditioner compressors.

there is no mechanical reason for any internal combustion engine to have a fuel source other than LPG or natural gas.

however there are dual fuel systems & conversion kits that allow you to run either LPG or gasoline, depending on whats most available to you. I've personally seen a few farmers pickups set up that way.

yes, the propane needs to be vaporized before use. You can design a lpg car that has a large tank and small engine that can start on lp, but, a small tank large enge needs to have some way the get enough propane to vaporize before the engine runs long enough to get hot enough to vaporize the lpg.