The wing mounted refuelling pods were perfectly capable of being fitted. However, financial limitations and lobbying by interested parties robbed them (and the money) for the various VC10 rebuilds.
OAP - are you sure about that? The long faces and sucking of teeth from the Marshalls team who came to Brize to view a VC10K flight indicated that the design was far from mature. Not only would the TriStar have needed a different pod (Mk32A) with a longer hose (79ft) than the VC10K, but also the internal fuel pumping pressure was inadequate to meet the flow rate and 50 psi end pressure stated in the specification. Even after replacing the refuel/defuel line with one of larger bore and greater strength, it would have taken further development investment to establish whether the specified offload rates could have been met under all required conditions. So the proposed plan was canned - a great pity as it would have provided the RAF with a very capable tanker.
Incidentally, as you're no doubt aware, the old wives' tale about the TriStar wing being unable to carry a pod due to structural and aerodynamic reasons is completely untrue - it could have carried wing pods quite easily.
I imagine that the MoD saw the VC10K4 and VC10C1K as less-risky alternatives? But whether or not that was actually true....
Xercules, I make no secret of the fact that I am involved in the upgrade programme for the A310 MCS. The only 'vested interest' I might have is frustration at those who have wasted considerable time and effort in attempting to reinvent the wheel to create an MPS of their own when an acceptable solution was already at hand - in the same company.