https://www.gov.uk/towing-with-car
...and a quote:
What you can tow
The rules on what you can tow are different depending on when you passed your driving test.
View your driving licence information to see if you’re allowed to tow.
Licences issued from 1 January 1997
If you passed your car driving test on or after 1 January 1997 you can:
drive a car or van up to 3,500kg maximum authorised mass (MAM) towing a trailer of up to 750kg MAM
tow a trailer over 750kg MAM as long as the combined MAM of the trailer and towing vehicle is no more than 3,500kg
MAM is the limit on how much the vehicle can weigh when it’s loaded.
You have to pass the car and trailer driving test if you want to tow anything heavier.
Licences issued before 1 January 1997
If you passed your car test before 1 January 1997 you’re usually allowed to drive a vehicle and trailer combination up to 8,250kg MAM. View your driving licence information to check.
You’re also allowed to drive a minibus with a trailer over 750kg MAM.