First and foremost, you need to let the DVLA know your vehicle is no longer a van. You can do this by changing the vehicle type on the V5C registration document to "Motor Caravan" and including a list of what has been done to convert it. Its quite easy if your converted van meets the requirement of the DVLA.

For a vehicle to be recognised as being converted to a motor caravan it must meet four categories. For the DVLA to change the body type, the vehicle must have the external permanent features described in the DVLA guidance for converting a vehicle into a motor caravan. “Motor Caravan” means a special purposes passenger car constructed to include living accommodation which contains at least the following equipment: Seats and table, sleeping accommodation (which may be converted from the seats), cooking facilities and storage facilities. The equipment must be rigidly fixed to the living compartment. However, the table may be designed to be easily removable. As it is no longer a goods vehicle it will qualify for Class 4 MoTs. However, as it is derived from a van you must adhere to van speed limits and these do not change accordingly. The DVLA may request to inspect the vehicle before allowing a change of body type.