Appreciate all the comments, had a list from one of the dealers of insurance companies to try said we can get dealers discount (all the ones I tried said they're not allowed to now). Had already tried A-plan as a camper quote £928 as a day van £586??? As for getting too hot very rarely go south in summer. We've done it before in small campers & large well insulated A class motorhomes & there wasn't much difference but fully understand your view. We wouldn't be sitting inside when the weather is good.
We originally were going for a cheapish conversion, did think about buying van & getting it done (done a couple myself in the past & understand what's involved. As for quality of CamperKing I can see that they are built to a price, all of the motorhomes we've had I could find faults with. We have had 6 vans/motorhomes in the past 17 years since we retired & overall done many miles in Europe without costing us much. Some we lost money on others we sold at a profit.