If you've passed a driving test you can drive on your test pass certificate until you get your proper licence through.
I believe the Army won't let you drive their vehicles though. At least they wouldn't let me drive a minibus until my new licence came through even though I'd been taught by the same regiment and been tested by an Army examiner. That might have just been our MT SSgt being a jobsworth though.
Wait till you get your full licence (I assume you've sent your provisional off), otherwise you are "driving other than in accordance with a licence". This is why people have been done for driving when they havn't renewed their licence (even though they've passed a driving test), it's still classed as driving with no current licence.
Yeah they've retained my provisional to be sent off to the DVLA. Going to play it safe I think and wait until either the DVLA have received my license to be processed or I actually receive my shiny pink card.