B+E car and trailer
If you passed your driving test after 1 January 1997 and have an ordinary category B (car) licence, you can drive either:

A vehicle up to 3.5 tonnes or 3,500 kilograms (kg) Maximum Authorised Mass (MAM) towing a trailer of up to 750kg MAM (with a combined weight of up to 4,250kg in total)

A trailer over 750kg MAM as long as it is no more than the unladen weight of the towing vehicle (with a combined weight of up to 3,500kg in total)
For anything heavier you need to take a category B+E driving test.