I'm not sure what the issue is here.
You update a fleet because you expect it to be in service for some time - simples!
Plans change, a defence review comes along and scraps that fleet - not ideal, but things like this happen, and the bean counters view of money is that once its spent (i.e. the money on the update) then that's history.
You now have a fleet of aircraft you no longer want. Do you:
a. Put them in storage for a rainy day - this actually costs you money, under the wonderful scheme introduced by a Mr G Brown.
b. Scrap them for pennies.
c. Sell them for what you can get - in this case £112M, which is at least more than pennies.
While option c might not go down too well with the tabloid readers, it is still preferable to options a and b!! A case of the best of the lesser of two (three) weevils!!
Edited to add that the article states that the £600M was spent over the last 5-10 years, when no doubt the fleet was thought to have a long term life!