The chassis plate on the vehicle (see table) states the maximum weights allowed - the Gross Vehicle Weight (GVW) and the Gross Combination Weight (GCW).
The vehicle handbook will either repeat what is on the chassis plate, or for convenience, might directly specify the maximum weight of trailer (eg 750kg) which is allowed to be towed. The V5C registration certificate often shows this too, under sections O1 and O2 (depending on whether trailer has brakes or not). Exceeding any of the above weights is likely to be construed as using a vehicle in a dangerous condition.
Where the sum of the maximum plated weights of the towing vehicle and of the trailer added together exceed the plated GCW of the towing vehicle, this is not a problem as long as the ‘actual’ weights of the vehicle and trailer (which may not be fully laden at the time) do not exceed the plated GCW.
Weight capacity of the trailer
A trailer manufacturer must decide the maximum weight the trailer can be loaded to, the Gross Vehicle Weight (GVW) of the trailer, and mark it on the trailer chassis plate. This cannot be exceeded. Sometimes the maximum axle weights are quoted instead and these must not be exceeded.