If you take along the correct forms, together with the payment (and MOT in my case) and are prepared to wait about 40 minutes, then they will issue you with the number plate form and tax disc there and then (well thats what happened at Bristol DVLA), although my trailer man wasn't too happy with the wait
Tim
PS A few bits had carelessly fallen off the car since the IVA but I didn't risk too many!! Its only a 5 minute inspection, they spend most of the time finding the engine number...