I disagree with a lot of the comments above, you’re clearly not going to be a testosterone fuelled, boy-racer in a van, just don’t drive like a pillock.

Respectfully that is just a total guess. As its the opposite conclusion, Hence why insurance is based on analysis of the statistics and the risks
These make him a very high risk drive driving a fairly substantial vehicle – in comparison to anything else he has driven which again exacerbates the risk.
The insurance is high because the risk is high.

can’t see the issue with the the weight or size

Harder to drive, poorer handling, less visibility, do more damage etc
YOu cannot see why this exacerbates the risk
I suspect it exacerbates it for folk who have been driving years never mind noobs.

. The problem is, I’m 19 with no driving experience (not strictly true, I’ve been learning and doing plenty of driving for the past 4 months, but as far as the insurers are concerned I’m utterly clueless).

This is the key to it all, you are clueless, you have no experience, you have never driven on your own before. The worst bit is you think you have a clue. That is the reason so many young and inexperienced drivers have/cause accidents.
If you call a person and tell them that they will actually laugh at you, then tell them you want to go to bike races in it! How far have you driven in one go? How do you expect to feel after racing?

When I was first driving my first car was a series landrover and that was cheaper to insure than a normal car despite being a tank that handles badly

i was guessing it’s the age rather than car type that’s the issue with price

even now I pay less to insure my van than my car
But landrover theft rates have pushed the cost of insuring my defender lots

Between myself and my partner the cheapest thing we insure his her 7.5t lorry

So for me Personaly size of vehicle does not seem to affect cost ,

So you think there is no risk difference between getting a new driver to drive a 7.5 ton truck or a mini car.

Size is not the only factor for insurance but it is an extra risk factor with a new driver.

I would try and get a quote from people like Adrian Flux or Lancaster ins. In your position I would get another family member to insure the Van. Get yourself a little run about and get some experience under your belt. See if you can be added to the Van policy as a second driver and only drive it on holiday ie as a legit infrequent second driver. We’ve all been through this time experience and not claiming is your friend!