Well we could discuss this for weeks without any useful purpose (and it would be interesting to actually see the formula in all its complexities written down) but like reading council reports and the like generally, maybe life's too short.
But in general terms, for a cost-based approach to fare-setting, then obviously the more *specific* the numbers involved the better - after all, index/inflation numbers are just generalisations used for various purposes, whether in terms of prices generally (what's usually reported as the headline 'inflation' figure), or more specific indexes, such as house price inflation (or, of course, deflation in that it's not unusual for house prices to go down).
So the AA figures were simply about motorists' costs inflation, and not specific to the cab trade.
The likes of licensing fees are relatively easy, since they're decided by each council, and each council in turn sets the fares.
But, as you say, we all have different costs, and if the methodology was taken to its ultimate conclusion then the council would set different fares for every driver
Which is obviously unrealistic.