There is so much ****e spouted about this subject, it is ridiculous.

The law is really simple. If you have category B+E on your driving licence, then you can drive a vehicle with a max gross weight of 3500kg and any trailer provided that;
A. The MAM of the trailer does not exceed the vehicle's max towing weight.
B. The combined weight of vehicle & trailer does not exceed tow vehicles max GTW.
C. If trailer weighs more than 750kg, the trailer has brakes that operate on all axles.
If you do not have B+E on your licence, things are slightly more complicated.
You can tow any trailer up to 750kg behind any vehicle with a MGW of up to 3500kg. You can also tow more than 750kg provided that;
A. The weight of the trailer does not exceed the unladen weight of the towing vehicle.
B. The trailer does not exceed the max towing capacity of the towing vehicle.
C. The total combined weight of the tow vehicle and trailer do not exceed 3500kg or the max GTW of the tow vehicle if that is lower than 3500kg.
The law makes no distinction between trailers, caravans and horseboxes, and there are no limits on number of axles.
The maximum dimensions for trailers you can tow on B or B+E on the road are;
Max length: 7m (excluding coupling & draw bar)
Max width: 2.55m (2.3m before 2010)
Max length of combination: 18m
Max overhang of load over rear of trailer: 3.05m
I regularly tow loads of up to 3500kg (4000kg on last Defender with coupled brake system) and up to 30ft in length. I also occasionally tow under tacho regs, which is a whole different headache.