The RAF will still need a centreline hose for A400M, Nimrod '2000', MR2 and R1 and C130....
You don't really need an A330 upper deck door or freight floor as there is already ample space underfloor. Very expensive to modify.
Boom would require a fair bit of operator training and practice to become proficient. Bit of a bugger if you've only got E-3s to joust - unless the C-17A AAR clearance is taken on board by the RAF. Or, of course, if another NATO player with a receptacle is around.
Boom would, however, enhance interoperability and would also be feasible for UAV AAR. If anyone thinks that they would trust one of the little sods to make contact on a drogue, think again!
As for tanker-tanker refuelling - definitely a nice to have rather than a 'must have'. Probably easier to use the boom with the A330 inertia and thrust control system, rather than probe-and-drogue. TriStar probe was very draggy and very noisy on the flightdeck, so was $hitcanned.
And it was ME, not the OzAF CAS who coined the term "Poms are f*****g idiots" regarding the absurdity of PFI! Perhaps he read it on PPRuNe?