With the cockpit commonality between the late A300s, A310s and A330s, perhaps the strategy should be to accept a mixed fleet (would it really be any more problematic than the present VC10 C1K/K3/K4 and TriStar C1, C1K, C2 fleet?) and to buy what's available as it becomes available. You could even buy a second hand A340 as Blair Force one. You'd probably want to avoid one-offs engine-wise, but otherwise.....
A phased, incremental acquisition of this sort would probably remain within the £500 m p.a figure, too.
Three A310s sounds like a darned good start to me, and if the 'non-standard' jet is available for lease, perhaps that would be a useful short term transport/trainer? You could always mix new-build and second hand jets.
See
http://airplanes.glo-con.com/images/AIR1_48_s.jpg