Van insurance is costly. My van is officially group 3 but costs more to insure than a group 10 Golf GTI. (im 24 with 3 years NCB).
Derv cars tend to be of a lower group than the equivalent petrol model so if budget stretches go for a derv as they tend to cost more in the first place.
If it's your car you cannot get insured as a named driver on a parents policy – if you have an incident they won't pay up and you'll be fined/charged for being uninsured. By driving the car more than the policy owner, this becomes the case.
If you're set on a van, consider a Citroen C15 1.5 n/a (non-turbo) diesel. It will cost about £600 to buy a half decent one and could be very cheap (relativly) to insure in comparison to other vans, maybe cheaper than a car too. Look into it. A friend has had 4 of these in succession and loves then for the simplicty/field car-esque reliability. He drive from Loughborough to N. Wales and back with a snapped clutch cable just by slamming the gear lever through and matching revs! Engine lasted another 5 months before needed a swap.