Jess, according to the news I heard at lunchtime, the grounding is because the work was supposed to have been done more quickly than it has. The grounding is supposed to allow the work to proceed more quickly.
I would guess that the grounding will have two effects. Airframes will be available for Mods rather than being diverted to essential tasks.
The groundcrew who would be servicing the in-service jets can be deployed for a surge maintenance programme.
Just a guess.
Len, I would have thought they will be deploye dback in theatre once modified. The same news item said that the capability would be maintained by the use of other UK and allied assets.
One might ask why wait another 2 weeks or so. I would guess it is to allow time for those other assets to be deployed in to theatre.