LPG has one other benefit which may apply to your car - once converted you can re-register it as an alternative fuel vehicle which means you save on annual road tax - probably an amount which would pay for perhaps 2-3 tanks of petrol, or 3-6 tanks of LPG.
You also get to drive in London's congestion charge zone for free. But as you are 450 miles away thats maybe not such a benefit.
Not sure what you mean about the converted 318 near you, do you mean you would strip the LPG off and add it to yours ? Doesn't an installation have to be approved by someone, not sure on this though.
There are rumours (nothing stronger than that though) of a change in the taxation for LPG, and these changes usually result in uk.gov removing more of our money not the other way round.
Can't help thinking that if I ran a 328i, 330i or even an M3 then this would make sense, but I wouldn't convert a car with reasonable MPG like your 318is. As you like the iS and are torn about letting it go, could you SORN it instead and run a smoker diesel hatch as an everyday car - bangernomics style ? Or as an alternative get your 335 converted and run that instead.
Or for top spannering swap the 335 engine into the iS - but keep everything else looking standard.
