That is the case with many vans. It is possible to buy a 6 berth van with less seat belts.
I also found this.
For vehicles built up to Oxtober 2007 there was no legal requirement to have seat belts fitted to side-facing seats or seats that make up the accommodation area in motor caravans. Regulation 46 of the Road Vehicles (Construction and Use) Regulations 1986, as amended, states motor caravans first used on or after 1st April 1982 but before 1 October 1988 shall be equipped with anchorage points for the driver's seat and specified passenger seat (if any); and for motor caravans first used on or after 1st October 1988 shall be equipped with anchorage points for the driver's seat and any forward-facing front seat. You can download a copy of the SI at
http://www.opsi.gov.uk/si/si2001/20011043.htm. However, this did not preclude manufacturers fitting seat belts to forward facing or rearward facing seats within the accommodation area if they wished to do so. Where seat belts are fitted they must be worn. Seats in the rear of a campervan/motorhome did not, prior to October 2007, require seatbelts (whether forward, rearward or sideways facing) and it is not illegal to carry unrestrained passengers in them while travelling, providing the vehicle is not overloaded. It is not something we would recommend, however. Although current seat belt wearing regulations do not currently prohibit carrying more passengers in vehicles than there are seat belts available, the police may prosecute drivers for carrying passengers in a manner that may injure someone. We would advise that no-one should be carried in any unbelted seat in the rear of a motorhome. An EU Directive (2005/40/EC) on the installation of seat belts required that from 20 October 2007 new vehicles have to have seat belts fitted on all seats except those seats intended solely for use when the vehicle is stationary. Where seat belts are fitted, from May 2009, the seat belt wearing Directive will prevent more passengers being carried than there are seat belts in the rear of vehicles. This will mean that from May 2009, in any vehicle of whatever age, where seat belts are fitted in the rear, more passengers may not be carried in the rear than there are seat belts available. The critical point for owners of older motorhomes is that it will not become illegal to carry passengers in the rear, provided that no seatbelts are fitted to any seats behind the driver and front passenger seats. Owners of any motorhome that has belts fitted to any seat in the rear will need to be aware that, from May 2009, it will be illegal to carry passengers in any unbelted seat.
I think the last two lines is the most relevant to you original question.
