Check to see if you have the STATIC aircraft option enabled. This will add static aircraft at a handful of gates, and these gates will not be available for AI planes or the user. Can you let me know which of the following two files is installed in your UK2000 Scenery/UK2000 Aberdeen Xtreme/Scenery folder:-
EGPD_AFX_UK2xGEN.bgl
EGPD_AFX_UK2xTRM.bgl