There are lots of Volvo T5's, Audi's and the like out there on gas and tuned in to the mid 300bhp's, so as long as the installer knows his onions there really shouldn't be any issues with going down the LPG route and you'll save lots of money on fuel.
I'm pretty sure there's a good LPG installer out Malton way (check on the RR forums) and that's not too far from you...