Car (from the VIN plate & V5C)
Load & trailer
The four numbers that decide everything
Every tow setup is governed by plated limits, most of them stamped on the VIN plate in the door aperture or under the bonnet. Maximum authorised mass (MAM, also called GVW) is the most the car may weigh including everyone and everything in it, plus the noseweight pressing down on the towball. Gross train weight (GTW) is the most the car and loaded trailer may weigh combined. Maximum braked towing capacity limits the trailer itself. And the noseweight limit caps the download on the towball. Exceed any one of them and the setup is illegal and uninsured, regardless of how the others look.
Payload disappears faster than people expect
Payload is simply MAM minus kerb weight, and on many cars it's only 400 to 600 kg. Four adults, a loaded boot and 75 kg of noseweight can consume all of it before the trailer is even considered. Note that kerb weight definitions vary: some manufacturers include a 75 kg driver and some don't, so if you're close to a limit, a public weighbridge gives the honest answer for a few pounds.
The 85% guideline
Separate from the legal limits, caravan and towing organisations recommend the loaded trailer weigh no more than 85% of the car's kerb weight for novice towers, and never more than 100%. This is about stability, not law: the heavier the trailer relative to the car, the more a snake (trailer sway) can overpower the car's ability to damp it. Heavier, longer-wheelbase cars tow more forgivingly, which is why the same 1500 kg caravan feels planted behind a large estate and lively behind a small SUV.
Licences and other checks
UK driving licence rules for towing changed in December 2021, and most car licences now permit heavier combinations than before, but the rules depend on when your licence was issued and its categories, so check your own entitlement on the government's official checker rather than relying on rules of thumb. Beyond the licence: trailers over 750 kg generally need their own brakes, noseweight should sit near the top of the allowed range for stability (a set of bathroom scales and a length of pipe measures it), and tyre pressures usually need the car's fully-laden settings when towing.