I don’t understand how ‘case by case’ basis was ever supposed to work in Equality Law.
Either you are excluding for a reason that has nothing to do with the PC, so no need to refer to case by case basis.
(Example: I am not a Catholic but my child can’t go to the Catholic school because we are outside the catchment area)
Or you are using the exclusion, but not consistently, which seems like discrimination.
(Example: the Catholic school allows some non Catholics, but does not explain the criteria for acceptance, explaining that decisions are made on a case by case basis).