ACR CROSSED RWY HOLD LINE AT BHM AFTER RECEIVING ATC TKOF CLRNC; TWR CHANGED CLRNC TO HOLD SHORT; TFC ON FINAL WAS ISSUED GAR.

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WE WERE CLRED FOR TKOF. THERE WAS A CESSNA ON FINAL BTWN APPROX 2-3 MI. KNOWING THE CESSNA WAS SLOW ON FINAL; THERE SEEMED TO BE PLENTY OF SPACING FOR US TO GET AIRBORNE BEFORE HE CROSSED THE THRESHOLD. HOWEVER; HEARING OUR CLRNC FOR TKOF; THE CESSNA ASKED IF HE SHOULD GO AROUND. AT THAT POINT THE TWR CTLR (WHO WAS ALSO WORKING GND CTL) TOLD US TO HOLD SHORT. WE ADVISED HIM THAT WE HAD ALREADY CROSSED THE HOLD SHORT LINE AND WERE UNABLE TO HOLD SHORT. THE TWR CTLR THEN SENT THE CESSNA AROUND WITH GUIDANCE TO TURN TO DOWNWIND. WE WERE SUBSEQUENTLY CLRED FOR TKOF AND DEPARTED. IN MY OPINION; PART OF THE PROB WAS THE TWR CTLR ALSO WORKING GND CTL. PRIOR TO CLRING US FOR DEP; HE HAD CLRED SEVERAL ACFT TO TAXI. WHEN THE CESSNA ASKED FOR GUIDANCE; I THINK THE TWR CTLR REACTED BY TRYING TO KEEP THE RWY ENVIRONMENT CLR. UNFORTUNATELY; IT WAS TOO LATE. ONCE WE STOPPED PER HIS INSTRUCTIONS; IT WAS THEN TOO LATE FOR US TO CONTINUE SAFELY AND HE HAD TO SEND THE CESSNA AROUND. I REALIZE THAT TWRS DO GO TO MINIMUM MANNING AT TIMES; BUT BY THIS TIME THERE SHOULD BE A SEPARATE TWR CTLR.