Well assuming that it's taxed, insured & MOT'd in the first place, which any vehicle needs to be to even be parked on the road, let alone moved along it, there are various laws that prevent the use of roadworthy vehicles on the road. For example, whilst you are allowed to tow a broken down vehicle along the road in an emergency (i.e to get it off of the road and to a place of safety), you're not allowed to intentionally move it on a planned journey from A to B in that way. So I can't therefore see how can you intentionally push an unroadworthy vehicle along the carriageway, regardless of whether you're driving it, or the keys are in the ignition.
I doubt very much that a vehicle without a rear subframe and therefore rear wheels, brakes, suspension etc could ever be deemed as roadworthy and I think you would struggle to claim that a car with no rear subby had just broken down and that you are just pushing it to a place of safety. " Yes officer, I was just driving along and then suddenly there was this bang, so I got out and my rear subby, wheels brakes suspension etc had fallen off with no trace..... luckily enough I happened to have these dollys/castors in the boot, so I could push it to my mates house" 
Edited by AVV IT, 15 May 2013 - 09:44 PM.