Copied from
http://www.dft.gov.uk/pgr/roads/vehicles/vssafety/requirementsfortrailersWeight
In the case of light trailers, that is less than 3500kg maximum laden weight, there is not any specified relationship in UK law between the weight of the towing vehicle and the weight of the trailer.
For M1 category vehicles (motor vehicles used for the carriage of passengers and comprising not more than eight seats in addition to the driver's seat) the maximum permissible trailer weight is quoted by the vehicle manufacturer. Alternatively, the vehicle manufacturer may provide a maximum gross train weight (the laden weight of the trailer plus the laden weight of the towing vehicle). If this is exceeded it is possible that the Courts or Insurance Companies may take the view that this constitutes a danger.
The maximum laden weight of a trailer which may be towed by a light goods vehicle depends on both the stated gross train weight of the towing vehicle (GTW) and the vehicle manufacturer's recommended maximum permissible trailer weight. Neither the maximum permissible trailer weight or the maximum gross train weight (the laden weight of the trailer plus the laden weight of the towing vehicle) should be exceeded.
It is possible that the stated gross train weight is less than the sum of the stated maximum permissible laden weight of the towing vehicle and the stated maximum permissible laden trailer weight. In this case the towing vehicle and the trailer must be loaded such that each does not exceed its individual maximum limit and the sum of both does not exceed the maximum gross train weight.
It is not a requirement to display a notice of the unladen weight of the trailer or the towing vehicle, unless the towing vehicle is either a motor tractor or a locomotive, as defined in the Road Vehicles (Construction and Use) Regulations 1986.