El Bunto
I think one of the reasons is that the MOD is wholly incapable of buying anything off the shelf and notoriously financially incontinent. Everything has to come with the maximum bells and whistles and bespoke to the highest level. Look at the fiascos of Nimrod AEW3, Nimord 2000, even TriStars converted rather than buy the KC10 off the shelf or even three different versions of VC10 tankers. Then you have the A330 where the aircraft is the right one but the PFI funding is insane. Sometimes all you need is a leased A321 outside of all that madness?