Vans are taxed differently to cars, with drivers paying a flat rate that's governed by the age of their vehicle: vans.honestjohn.co.uk/van-tax/budget-2018-how-much.../

Assuming you need a panel van, something like the Ford Transit Connect would meet your requirements. Payloads range from 600 - 1000kg while fuel economy for the diesel and petrol sit in the mid-40s.

I would also add the Volkswagen Caddy to your list. It's easy to drive, comfortable and will carry up to 900kg.