I can't be absolutely specific because I don't have the document in front of me, but I recall that recently there was some doubt about the use of 27L for cat 3 approaches because of a suspected bending of the localiser beam at a late stage of the approach.
Despite the runway recently being upgraded back to cat 3 after a period as a cat 1 runway, I recall a nubrief item saying that BA aircraft should continue to operate only to cat 1 minima on this particular runway until further investigation of the suspected bending had taken place.
This would perhaps explain the request for 9L in the original case in question.