All manufacturers publish recommended maximum towing weights, braked and unbaked, for their vehicles though I don't believe there's any agreed methodology for arriving at them. Different engines and transmissions can make a lot of difference within the same range - from 1800kg to 2500kg on one current car
The gross vehicle weight and the gross train weight, vehicle plus trailer, will be on the VIN plate.
It is very unclear to me which law(s) you break by exceeding manufacturers recommendation but if you have an accident it will be persuasive with both the old bill and your insurer as to whether they cuff you or cough up.
For decades the caravan club has suggested cars should not to tow above 85% of their weight, and definitely not more than 100%. There's no law behind this but single axle trailers are susceptible to snaking while I defy anyone to provoke that with a twin axle Ifor Williams