Is the "confirmed allocation" string in "A Scandal in Belgravia" a real airline practice?
No

Based on my research for string confirmed allocation, there were no references found related to airlines or their practice.

However, the background scenario about the conversation Sherlock was having, something like the following had happened (references: this and this and this and this and this):

(my paraphrasing, not actually a quote)


  Sherlock knows that 007 is a flight of Bond Air which is leaving for
  Baltimore tomorrow at 6:30pm. He is aware of a terrorist plot of
  having dead bodies on an airplane (14 seats allocations) so that no one actually dies. The
  first 0 in flight number indicates that zero causalities will happen.


P.S.: This question is not about aviation but movies.