I know its an old post but when you apply for an o licence you have to apply for 1 truck and 1 trailer unless you are only going to run a rigid.
An owner driver with a unit still needs to have at least one trailer specified on his o licence.
The trailer operator doesnt need any type of licence , just a telephone.
The o license holder is also responsible for the trailer. if its badly maintained its you who gets done or gets a GV9 (PG9)