Also...
Private companies do make a profit from regulary importing/exporting aircraft (757's in my experience) every 6 months to make the best use of cyclic travel patterns.
The RAF has a history of buying second-hand aircraft - VC10 and Tristar to name but two. And the RAF has been known to sell/rent/loan its aircraft out - Italian F3s.
The purchase of two low-value but useable freight and passenger capable aircraft for short term use is entirely reasonable and properly economic sense.
If they are successfully used, I can see no resaon why the RAF wouldnt buy more 146/RJ aircraft to fill some transport gaps til their stretched contracts come to fruition.
It's all been done before (but 'they' then kept the short-term buys for dozens of years! Bucc's)