because no one knows who owns them to give them away....
It is, or was for all the years I spent in MoD, a fundamental principle that every single item in the inventory had a designated owner, who determined its disposition from the day it was a Concept to the day it was disposed of. In practice, it was the job of the person who made materiel and financial provision, and acted as the interface between OR/DEC and procurers; a role nowadays shared between ILS and Requirement Managers. (And probably falls between the crack). Requests for such kit were a pleasant diversion and there was a well worn process for making sure the recipient knew the boundaries of what we would and wouldn't provide; now and in the future. Any prep work, like stripping out kit they weren't allowed, was tasked to an apprentice training centre. And there's the rub, we don't have such things now. We pay industry a fortune and lose flexibility.
Admittedly, a complete aircraft would be a bigger issue, partly because MoD would regard it as a
PR coup and involve the press office and senior staffs who wanted pictures in the press. Not sure who'll turn up at handover in Moray given it has had to be paid for. A lost opportunity, especially after the screw ups that have decimated Kinloss. As they say, MoD know the cost but not the value........