Doesn't the road tax apply only when the car is still in UK? And all this you posted from DVLA applies only when it's sold to a person living in UK?
In my country there would be no problem if I had a foreign car registered to a person in UK and I have only purchase agreement. I can even sell this car to another person giving him my purchase agreement with the owner on a logbook as well as new agreement between me and that new owner. That's why I'm so surprised you cannot do this in Malta. Also I could insure my car while it was not registered to my name, hence my disappointment today when insurance company said it is no longer possible (said that there were some changes in regulations just a few months ago and before it was no problem to insure a car just giving my ID and showing the purchase agreement).
So to sum up: I can't sell the car here, I can't insure it and I can only register it when I become resident (I need to be employed for that right?) bu will have to pay the huge tax.
Martin