Evalu8r has about the best option.
The "RAF" has never made any good or long lifed item from messing about with Mods to develop some new roles. All attempts have resulted in over-weight and over-stressed airframes that needed to be carefully nurtured to preserve the frame's life, and the crew's.
The only cash-effective answer is to start with a new frame built for the job.
Some areas of the forces have filled their gaps with smaller fuel efficient frames (B350's) and yes there is some new larger frames about.
But trying to reinvent the old "dreamship" from scrapped vans is a pure pipedream belonging to fans of BA Barracus. (And even his team didn't think of doing that to an aircraft!)
Of course, another obstacle SHOULD be the new MAA...if it ever gets its Sh1t together. The MAA should not be able to approve another bVggered-about botch job for future use.