I would be careful of this one - assume the rights of the owner and you're in the territory of theft should you remove the car or anything from it.
It may help to try Murph's suggestion and contact the DVLA as a first step. You might find that they're slightly more reluctant to provide this service in the absence of a fee - it they do it at all nowadays given the litigious society we live in and the disinclination for 'civil servants' to expose their arses to anything much more than a seat cushion.
In any event, you will have to show due diligence in your attempts to contact the owner through the last registered keeper by placing a notice in the local paper/ making local enquires/ contacting the DVLA and so on, steps that will perhaps make this more trouble than it's worth.
Of course the mere fact of informing anyone in authority of the existence of an untaxed (and uninsured vehicle) vehicle parked up can be a game changer in this authoritarian country - as the vehicle may be seized and the former keeper pursued for unpaid taxes and any other liability that applies.