Interesting discussion here - the project's going well, but there's still a huge amount to do
As well as getting voyager ready to operate with RAF platforms, there is also aircraft certification to achieve, which will be challenging for the first of type aircraft - the 2 point and 3 point variants. I think these are the primary reasons the aircraft will be at Boscombe for the Summer and Autumn.
It is also worth remembering that AirTanker Services has huge challenges ahead as an airline start-up. As well as achieving civilian ops and eng regulatory approvals, the company has got to recruit, train and (probably most importantly) retain flight crew, engineering and ground ops personnel in competition with the likes of Virgin (10 x A330 inbound) and a potentially strengthening industry. Finally, integrating a new airline into the RAF's legacy operations at Brize and elsewhere will be no mean feat.