The fact that you want to insure it for personal use doesn't change the fact that it is a commercial vehicle. A broker who does not specialise in commercial vehicles is probably only going to be able to quote for more common models, which seems to be what you have experienced.

As a separate issue, I suspect that trying to insure what is essentially a small truck for personal use could well prove tricky. Things may be different now, but in the early 90's I had a small Suzuki van, the predecessor to the Supercarry. Despite having an 800cc engine, it cost a lot more for me to insure it for personal use than a car of a comparable size. When I asked, I was told that insurers are extremely suspicious of someone insuring a commercial vehicle for personal use (possibly thinking that I had a sideline as a 'man with a van').