AFAIK you do not need a licence to be a plumber. If you have the dosh, put in for the cancellation. Driving courses can be hard to come by or just fall in your lap for a job/tour, best bet is to do it now while all the training is fresh!
You don't need it as a plumber but it is well work doing off your own back. At Gib you will be duty wheels rather than stagging on. Also it's better driving a 432 or Spartan rather than being stuck in the back when it's 50 degrees and you are wearing bodyarmour.
Im a civvy Plumber ,then joined up and became a A2 chippy ,didnt get my driving licence cos it was not needed for my trade ,so get it before you go in , or get stuck in the rut of just moaning every weekend how sh!t the place is and going on the lash ,like I did and spending all your cash so you are all ways skint