MDD did offer the RAF a 'C-17K' with R/R RB211-535E4s, a probe to supplement the USAF
style refueling receptacle, in the mid 90's.
At the time, defence spending was very much going down, the C-17 also had a bit of a problematical start too, now of course it's more mature and pretty essential.
Personally, I would have preferred 10-20 of those 'C-17Ks', then refurbish some of the C-130K/C.3s, replacing the more worn out ones with these C-17s, then buying C-130Js rather later than actually happened, letting some other poor sod have all the hassle of the C-130Js very protracted clearance for full service.
And/or getting A400Ms later.
However, it does appear likely that the current aircraft will be brought, possibly increased to 6 or 12 aircraft.
Which should not have any bearing on the A400M buy.
Yes a rather mixed fleet, however the RAF will have C-130Js for tactical work, the A400Ms for tactical/strategic airlift, the C-17s for strategic.
Not ideal from a cost point of view, but the RAF won't want for decent airlift anymore.