currently £180 for a commercial, will be a flat rate as it is taxed as a light goods vehicle not a private/light goods.
the link here will take you to dvla website where you can check the correct rate for your vehicle against a registration number and a make:
DVLA Vehicle Licensing Online | Vehicle Enquiry
In one of the other sections (not sure if disco or defender) there is a similar thread which states for vehicles made after 2001, dvla will not change the taxation class regardless of what modifications are made (i.e. van change to car or vice versa). would be ideal if true for changing a commercial into a car, not that hard to do, save a bundle.