You do not have to report or pay anything to HMRC if your van is only used for business journeys or as a pool van. A van solely used for work purposes, includes "insignificant private use" such as a detour to the shops for a coffee on your way to work, or a visit to the dentist during business hours.
What is a van?
"A vehicle primarily constructed for the conveyance of goods or burden• A gross vehicle weight - fully laden - not exceeding 3,500kg"
This covers panel vans with two or three front seats, chassis cabs fitted out with conversions, two-seat commercial SUVs and pick-up trucks.
You need to be careful with double-cab vans (with a second row of seats behind the front row)
Double check the van's V5C registration document for its classification. If the V5 says N1 or N2, then it's a van, but if it reads M1 or M2, then it must be taxed as a car.
A five-seat double cab pick-up must have a payload weight in excess of one tonne to be classified as a light commercial vehicle (LCV)