I would think it would be very impractical to change to LP. It takes a huge tank. The engine would need the propane as a liquid - not as a gas as the other appliances in the coach need. The tank must be placed in an open bottom compartment. Then the fuel gauges would need changing. There are some places where propane cannot by taken. Not sure about ferries and tunnels. That would need research. The cost of conversion would seem to be more that buying one built for LPG.
Back in the 70s gas crunch, I converted my pickup to run on propane (or gas). I had an 80 gallon tank crossways in the bed. It took up a lot of room, but with the propane and gas together I was able to get to the Lake Tahoe ski areas and back from San Jose. For a motorhome you would need a much larger tank, and even the 80 gallon unit I had would be hard to locate in a motorhome.
Back in the seventies, I worked for a while servicing fork lift trucks. I installed some propane converter kits for companies that wanted to use their existing trucks indoors. In all cases the customer was unhappy about the power reduction using propane. There was about a 1/3 loss in power. The engine use of propane doesn't require liquid, works the same as propane appliances.
Here is one source for LPG conversion. I would think that Canada might offer some opportunities too - they use LPG in vehicles much more extensively there than the USA. I assume you could drive the coach there and then export to the UK from there as well.