Oxford is outside CAS, unfortunately with NO radar service. I accept this situation is appalling and needs very urgently to be changed.
When you join CAS you will be separated from aircraft within CAS.
My guess would be that you were cleared to join CAS beneath a departure from GW, which would be at min stack level, either FL70 or 80, depending on the pressure, tracking towards CPT.
The next available level to join CAS beneath min stack level is altitude 6000 feet.
I hope this answers your question.