I seem to recall the RAF currently operating Vickers (VC-10) [15], de Havilland/Hawker Siddely (Nimrod) [15], and Lockheed (Tristar) [9] as large jet aircraft... vs 13 Boeing [7 E-3, 6 C-17].
39:13 (a 3:1 ratio against Boeing)...not quite a "Boeing-centric" RAF large jet fleet, now is it?
As for your "don't buy A400M (or anything but C-130) because C-130s are our only current large prop aircraft" "idea"... if the RAF thought they way you want them to, they would never have bought any aircraft from any manufacturer whose aircraft they weren't already operating when the RAF was formed (1918).
A really bad idea, I'd think.