Despite the inevitable Daily Mail-esque comments this is "normal business" for the MoD. As the majority of these vehicles were purchased "off the shelf" using CPF funding as UORs the MoD has only two options at the end of the campaign. Firstly, we could pay a large amount of money to move them back to the UK, provide a full Safety case (as mentoned above), pay for upgrades to make them compatible with non-Herrick kit and (the real killer) is that the MoD would be responsible for the full support costs for the vehicles from within its' budget - patently unaffordable unless the army give up capability elsewhere or receive an uplift in funds. Secondly, they leave them in Theatre and the FCO makes political capital out of "donating" them to the Afghan government - hmm, which is more likely?
There are aggravating factors; a lot of the UOR kit is made overseas - to keep it would cut UK manufacturers out of UK defence market for several years. Additionally, a lot of the Granby kit that did get brought back was disposed off very cheaply under the RAB restrictions, only to be very expensively (and publically) bought back a few years later...no politician wants those sort of headlines.