Please let's not raise the spectre of extra TriStars. We would be even more heavily dependant on Marshalls - who make BAE Systems look good. I remember a few years ago they quoted the cost of turning a $1m TriStar into a just a truck for the RAF as around £25m each.
I suspect that the Herrick airbridge will become the main motivator for the FSTA programme, with the AAR capability taking a back seat. It would be wrong, but not surprising, if this allowed some of the contractual obligations on AAR to be waived. The aircraft is already in just about the lowest AAR spec it is possible to have and the RAF could easily end up with another generation of botched tankers.
Contrast this programme with KC-X - the USAF's highest procurement priority. They know how crucial tankers are to any air campaign and their Generals seem to be interested in the actual capability and not just the cost.