I *think* that most low emission zones are more concerned with diesel than petrol so that's why London might like your LPG vehicle...because it's petrol at heart...
I might be wrong on that, but it might not be the LPG that means you're compliant...I think you have to have a very old vehicle that's petrol for it not to be compliant...
6. motor vehicles driven by, or carrying persons with serious mobility impairments, helpless or blind persons who have a severe disablement document marked with disability codes "aG", "H" or "Bl";
may well mean you are exempt with your units anyway