I understand the PFI contract also includes provisions that mean MOD had to pay AirTanker compensation if they use a charter aircraft for transport and not a Voyager (originally £8000 per flight plus £300 pre hour - index linked). Given MOD and ATI are entering the last ten years of the existing contract; surely there would be the possibility of changing the current terms, in return for financial compensation, to allow other aircraft to perform AAR roles which Voyager is not suitable for if there was a strategic incentive for MOD.
The Chinook HC.5s and the new ones on order are plumbed for AAR, as are the Jungly Merlins; I would have thought, especially in the special ops role, range extension is a clear value adding capability, particularly if there is a(n increased) need to rely on sovereign rather than "purple gas". Presumably the contract would prevent the acquisition of VARS thereby enabling refuelling F-35 and Merlin organically by a CSG, should sense prevail.