you can claim the full tanks quantity on your departure from UK.
You can claim only the fuel exported
in the tank, and then only the portion of it which has not been subject to a previous claim (obviously).
This kind of thing can get tricky with those syndicates in which an individual is allowed to pocket the duty drawback. What happens is that this chap fills up the tanks (obviously) and flies from Lydd to Le Touquet, and claims back say £100. He returns with a nearly full tank but that fuel is "blighted" because nobody can claim drawback on it on foreign trips, until it is all used up. When I used to rent out my plane, I had exactly this done by one chap (an instructor, perhaps unsuprisingly) and then I had to set a rule that drawbacks will be done by the company and not renters.