Per comments #4 and #5, I would disagree with the latter. Some manufacturers require upgrades for towing. An example is a Skoda who do not homologate the vehicle for towing unless it was order new with towing fitment or factory fitment. This is because if towing is required they fit extra bits including oil cooler and more powerful fan system amongst other things. Look at the VIN plate and see if it has two weights at the top then it is homologated, but if there is only one it is not and it is questionable at that point whether you would infringe the Type Approval. Prof will be along in a minute to confirm or destroy my comment!
Curiously our Passat Estate 2L 150TDi is essentially the same as most Skodas but with a different body and VW do not impose such restrictions. I would not however that without an oil cooler I have seen the oil temp as high as 116C towing against around 95-98C as the solo temp!
In the UK and EU for vehicle that were manufactured and sold after the Vehicle Construction and Use regulations came into force, All tow vehicles must have a Gross Train Weight (GTW) value on their data plate. If the value given is zero or it is not given, the particular vehicle is not legally permitted to tow a trailer of any size braked or unbraked. All the values on the data plate are upper limits
with no prescribed tolerance or upward allowance.
In the UK & EU any trailer with a Maximum Authorised Mass (MAM) exceeding 750kg must have brakes fitted
Where a GTW value is stated, then the combined measured weight
of the tow vehicle and trailer must not exceed the GTW value.
In most cases the vehicle manufacture will advise a practical "Braked Towed Weight Limit" or towed load value. This in itself is not a legal limit, but exceeding it would most likely be deemed to be having and unsafe vehicle, which has no prescribed values to define its application. In addition Insurance companies are likely to deem exceeding the manufacturer's specifications as misuse and might use it to void insurance cover.
The method using GTW-GVW to calculate to give a suggested maximum towed load limit simple and effective way of establishing a legal towed load limit for any vehicle. But you have to also appreciate the trailers nose load will increase the tow vehicles axle loads so you must allow for it when loading the outfit.