Axel
Sadly 2 simple answers:
There was no UK requirement for boom refuelling. The C17s were leased and we couldn't AAR them. The E3 has a probe for tanking from RAF tankers. The JSF variant that the UK were looking at would be probe and drogue equipped. Therefore the scrutineers in the MOD would not allow "gold plating" of the FSTA by allowing a boom and receptacle.
Which airline would be willing to lease, under the PFI irreducible spare capacity malarky, an airliner that would have additional weight (structural strengthening etc) for it to be capable of carrying a boom thereby eating into their profits.
Not defending the decision, which we all know flies in the face of coalition warfare and flexibility, just answering the question.