Living in the sticks here in Devon I'm going with the NFU for my new Kombi Highline 140 T32. Slightly more expensive at £500 (note I've declared 18K annual miles and had a prang this year which was stupidly my fault) but no amendment fees and NO surcharge for paying monthly so actually a very reasonable quote.
They used to insure my '98 Defender 110 and were very competitive adding my teenage sons to the policy as named drivers. Might be same with a van if you trust them with it.
Ring up your local branch and deal with a human who values your business. It's a mutual so if they perform they send you an annual cheque as your share of their profits, which in my case one year was about £25 if I remember rightly.
Brentacre were £460 for a single payment, admitted a included a £50 admin fee to start the policy and subsequent £45 admin fee to renew it which seem cheeky for me to give them my custom. Split over a deposit and 10 subsequent payments with 16 percent interest (!) it worked out at £505. As I've just taken out a loan with Sainsbury Bank for part of the purchase at 3 percent APR (save 0.1 percent with nectar card over 3.1 standard rate) that's even more cheeky.
Not sure if the Aviva app knows what speed a van should be doing on a dual carriageway but armed with paperwork mentioned on the speed limit thread on this forum I intend to drive at 70 when possible, the A303 is slow progress at the best of times!
So NFU it is and try them for other insurance too, they are very competitive.