Is buying a used LPG van, or new that needs converted, a good idea for fuel economy?

I drive 200 miles once a week from London to Devon and need to buy a van (used or new) that's smaller than a Citroen Dispatch. I have two children so it will need three seats. I'm looking at LPG in the...

Do the new van weight rules mean I can drive 4000kg on my standard licence?

Is the weight which you can drive on a standard license increasing from 3500kg to 4000kg? I can drive up to 3500kg at present, but any heavier and I need an operating license.

LPG - why hasn't it been more popular in the UK?

I often notice driver of Eastern European origin using the LPG pump at my usual filling station, what are your thought on why the fuel has not been more popular in the UK ?