The interactive website that partners The Car and Motorcycle Theory Test book
 

Provisional trailer entitlement

Since 1 January 1997, drivers can no longer take a test in a heavy vehicle with a trailer (for example, category C+E or D+E) unless they have first passed a test in the relevant vehicle (for example, category C or D).

This means that although drivers may have been driving a vehicle and trailer combination with ā€˜Lā€™ plates, they cannot take a trailer test with that vehicle until they have passed a test in the required category without a trailer attached.