Isn't there a more simple reason for keeping the dumbed-down Mk3's in the UK, such as that of commonality of aircraft being rotated through Afghanistan?
Whilst the 'dumbening' process is making the Mk3's almost the same as the Mk2/2a's they won't be exactly the same and restricting them to UK-based duties will free-up Mk2/2a's, which can then be added to the pool of aircraft rotated through Afghanistan, which will hopefully improve the serviceability of the whole fleet.
Whilst its easy get AATOB over this and start firing off letters to the Daily Wail decrying the MoD for wasting perfectly good new aircraft its probably quite important to bear in mind that the older Chinooks are the epitome of the 'Trigger's Brush' process. They may carry old serial numbers but I'd suggest there is very little left on them that was attached to those serial numbers straight out of the factory.