I looked into this in much detail, if you passed after 1997 but before the next change. (Can’t remember when that was). You can tow a 3500kg vehicle with a 750kg trailer
or
a combined weight of vehicle and trailer of 3500kg
for example if your car had a gross weight of 2295kg you would be able to tow 1205kg in trailer
be very careful with trailer choices because it also goes on the mam of the trailer, you can’t even tow a trailer plated at 3500kg empty
i built my own from a caravan chassis and because it’s old and doesn’t have a weight plate to capacity is taken from the max weight shown on the tyres,