Fixed cooking etc is the difference (inc water storage). I have copied this from
http://www.campervanlife.com/building/legal ;
"What makes a 'Motor Caravan'?
For a vehicle to qualify as a 'Motor Caravan' in the UK in the eyes of the DVLA the following permanent fixtures must be present:
Sleeping Accommodation
There must be a bed with a minimum length of 6ft or 180cms
The bed must be an integral part of the vehicle living accommodation area
The bed must be permanent or converted from seats (the bed can fold away during the day)
The bed fixtures must be secured directly to the vehicle floor and/or side walls, unless it is over the drivers cab compartment.
Door
There must be a horizontal sliding door or an outward opening rear or side door.
Seats and Tables
There must be a seating area for diners to sit around
The table can be fixed or detachable
The table must mount directly to the vehicle floor or side walls
The table mounting must be secured as a permanent feature, either bolted screwed or welded. The table itself can be detachable.
Seats must be secured directly to the vehicle floor and/or side walls
The seats must be secured as a permanent fixture, either bolted, riveted, screwed or welded
Permanently secured seating must be available for use at a table
Water Container
Note: DVLA do not state any requirements regarding water storage. However, most insurance companies state that the water tank should be onboard, or under the chassis. However, some insurance companies are happy with an external water container that can be moved, such as those used with a caravan.
The vehicle must have an onboard or external (e,g, under the chassis) water container
Note: The insurer Adrian Flux requires the water container to hold 6 gallons / 27 litres.
Storage
The vehicle must have at least one cupboard, locker or wardrobe
The cupboard must be an integral part of the living accommodation area
The cupboard must be a permanent feature, either bolted, riveted, screwed or welded
The cupboard must be secured directly to the vehicle floor and / or side walls
Cooking
The vehicle must have cooking facilities powered by fixed gas, electric hob or microwave oven
The cooking facilities must be secured directly to the vehicle floor or side wall
The cooking facilities must be a permanent feature, either bolted, riveted, screwed or welded
Gas and electric hobs must have a minimum or 2 cooking rings. Microwave ovens must have a power source (don't just fit one that can't be used)
Gas cooking facilities with remote fuel supplies must have the gas supply pipe permanently secured to the vehicle structure
Gas cooking facilities with remote fuel supplies should have the gas bottle, fuel reservoir secured to the vehicle structure
Outside
The vehicle must have at least one side window
New! Since 2011 the DVLA are now asking that the vehicle look like a motor caravan from the outside. The details are yet unclear what is required. More information will appear here when available."