Most drivers who passed their test before 1 January 1997 have licence categories B+E allowing them to drive a vehicle and trailer with a combined weight of up to 7 tonnes. The trailer must not exceed 3.5 tonnes.
Drivers who passed their test since 1 January 1997 are only allowed to drive a vehicle up to 3.5 tonnes (category B) and tow an unbraked trailer up to 750kg. Drivers may tow larger trailers over 750kg if the combined maximum authorised mass (MAM) does not exceed 3.5 tonnes and the gross MAM of the trailer does not exceed the unladen weight of the towing vehicle. To be able to tow combinations outside this ruling requires the passing of an additional test to obtain Categories B+E.
Drivers who passed their test after 19 January 2013, category B (car and small vehicle) can tow small trailers weighing no more than 750kg, a trailer over 750kg as long as the combined weight of the trailer and towing vehicle is no more than 3,500kg Maximum Authorised Mass (MAM)
If you want to tow a trailer weighing more than 750kg, when the combined weight of the towing vehicle and trailer is more than 3,500kg, you'll have to pass a further test and get B+E entitlement on your licence.
You'll then be able to tow trailers up to 3,500kg.
This information is not a definitive statement of law.
Please refer to your individual drivers' license to confirm your personal entitlement and seek advice from the DVLA and the professional bodies where necessary.
Details of the necessary licensing details and towing laws can be found on the UK Government Towing Guidlines