Commercial operators don't have to work within HMG finance and accounting regulations. These rules make the procurement process much, much slower than it could be, but they cannot simply be swept away.
On a practical basis your 10 aircraft would need at least 50 pilots and perhaps 150 engineers. Unlike commercial airlines the operation of large-jet transport aircraft is relatively niche part of the whole organization. People with the right experience to make a new aircraft type happen quickly simply cannot be conjured-up in such a short timescale without seriously denuding the output of the existing fleets. The problem is exacerbated by the hurdle of a step-change in technology caused by prolonging the life of the previous aircraft. I would suggest that, as the TriStar is so heavily tasked, most of the people would have to come from the VC10. Either way the pilots have no experience of 2-crew, EFIS or ETOPS. It's not quite the same as taking a bunch of 737 pilots and converting them to say a 757. Lack of relevant experience is not insurmountable but would need to be much more carefully managed than your claim would allow.
The other issue is regulatory. Military aircraft airworthiness is the responsibility of the MoD, not the CAA. The C-17 release-to-service was readily accepted as the lease was directly coupled to the DoD regulatory process. A new type of transport aircraft would have to issued with a MAR and all the necessary DAS mods cleared by Boscombe.
I assure you that it is highly unlikely that it could be done in 2 years, let alone just 9 months.