Hi Rob,
I run a 1997 3.9V8 on lpg and have had my cats removed for two years now. It has a pipe-craft custom side exit exhaust on (there in basildon Essex) and when they fitted the exhaust they also cut out my cats, I still have the lambda sensors in.
I have passed two mot's like this, if you present it on LPG it has to be tested on LPG.
I still have the cats in my lock up somewhere.
Andy