a vehicle with an unladen weight of 1.25 tonnes and a MAM of 2 tonnes coupled with a trailer with a MAM of 1.25 tonnes could be driven by the holder of a category B entitlement. This is because the MAM of the combination does not exceed 3.5 tonnes and also the MAM of the trailer does not exceed the unladen weight of the drawing vehicle

the same vehicle with an unladen weight of 1.25 tonnes and a MAM of 2 tonnes when coupled with a trailer with a MAM of 1.5 tonnes would fall within category B+E. This is because although the combined weight of the vehicle and trailer is within the 3.5 tonnes MAM limit, the MAM of the trailer is more than the unladen weight of the drawing vehicle

If you passed a car test on or after 1 January 1997 you're limited to vehicles up to 3.5 tonnes maximum authorised mass towing a trailer up to 750 kgs, or a vehicle and trailer combination up to 3.5 tonnes MAM providing the MAM of the trailer doesn't exceed the unladen mass of the towing vehicle. You'll need to pass an additional driving test in B+E if you wish to tow a caravan or trailer combination which exceeds these weight limits.