There are plenty of creative ways for the saving to appear in the budget but actually still be borne by the taxpayer.
For instance they might assume that the RAF maintenance and crewing is going to be "free" or at significantly reduced cost because the RAF would have to do this anyway if the aircraft were left unconverted—but of course the RAF do not have infinite capability to absorb extra work and eventually someone will have to account for it.
It may even be an RAF idea so they have an excuse to buy some more aircraft after converting one or two or three for the PM and friends.
Without seeing the real report it is not really possible to say either way what the numbers are and assuming the government has done it the proper way is up to you, but I am a little cautious in interpreting media reports.