Insuring a van before it's converted to camper

Hi all - Hoping there's not a better place to post this, but I have just purchased a van I need to insure, but haven't converted it yet. I can see there are specialist motorhome insurance providers, and that I need to inform an insurer if I modify my van. I'm a little unsure what motorhome insurance does for a campervan that van insurance doesn't (is it cheaper?) and what people's advice here is. Do I just take out the cheapest deal for the base vehicle for now (cheapest I can find is Swinton, followed by Admiral) and then inform them as the changes are made? And if so can I expect the prices to change much? Or is there such a difference between the two that people cancel existing insurance (cancellation fee hovers around £50) to switch once the conversion is done? Would really appreciate some advice.

Thanks!