Using an online quote and buy is basically impossible for most LPG cars, because it is a usually a modification, none of the online sites -I have used- allow the quote to proceed if there has been a modification, so you will not get your 10% online discount. Or cashback - if you use cashback websites.
My car on the other hand is sold from new as LPG dual fuel. The annoying thing for me is, that although it is not a modification, they do not list the Dual Fuel variants in the car finder lists on the insurance sites. And when you enter your Reg No. it just finds the car as the normal model, does not list it as Dual Fuel.
If they aren't allowing you to select a "factory standard" vehicle and it's causing you to lose out, then I'd be off to see my Trading Standards office about it.