Good Morning Philip,
Whilst the information Lutz has given may correspond with your theory that the allowable towed weight is equal to the cars kerbweight, the theory is wrong. The towed mass limit is not a simple figure based on the car kerbweight, but on a set of criteria that each car must match.
In practice the allowable towed masses can vary from zero to several times the cars own kerb weight depending on the model of car under test.
Whist it is legal to tow a trailer up to the manufactures stated limit, it is good practice to keep the trailer as light as possible and the car as heavy as possible. Doing this is part of a strategy to keep the towing experience as safe as possible, of equal importance is the load distribution within the car and caravan, maintenance of both vehicles and driving habits.