I think Joint Force is the only option. Engineering could be largely contracted out as it will mostly based in UK or MOBs abroad. Pilots could be a combination of RN and RAF retread pilots until the training stream gets going.
The RN is the only Service conducting officer Rear Crew training (ie Observers) so the logical thing would be to expand this. There is also another factor that argues for this route...career development. The requirement for officer rear crew will be relatively small so it makes no sense for the RAF to want to establish such a small cadre, whereas the RN can broaden such people into RW Obs and PWOs.
WSOps could come from both Services but career management would become an issue for the RAF.
All in all a bit of a mess really, but this will happen with so few airframes (projected). However, I expect the RAF to fight hard to own the full capability and muddle through.