Exactly right, Tiresome1.
I wanted to do a miles for upgrade last week, no availabilty, so paid cash.
Suddenly a whole lot of availability appeared, so not only for blues, but silvers as well :-)
On other other hand we got 4 very good seats at the time of purchase on another flight, when it would have cost someone else £160.
As my old contract law course lecturer used to say, "it's all a matter of obligations and entitlements."
Mind you, I don't like BA's policy of charging for seat selection in C/J, when you've paid a business fare, you should be able to select a seat at the time of booking IMHO, even if they block of the really good seats for the golds and blacks.