MyT... am with you on this. If you don't claim unaccompanied baggage you're allowed MMA to drive to your new posting, plus a return journey from your new post to your old one to pick up your gear and bring it to your new home. If you claim unaccompanied, you should be entitled to the cost of driving yourself to your new post, plus the cost of unaccompanied baggage (up to 3 triwall boxes or whatever it's changed to nowadays!)
Not sure what the score is on hiring a van now as I've always moved with a hugely practical car into which all of my personal belongings can fit, being sad, single and poor!