Indeed HH-that is exactly what happened in the case of Alcock vs CC Hampshire. Said gentleman failed selection but his female counterparts passed because of the gender fair test. The tribunal said this was direct discrimination which suggests to me that an objective assessment has been done.
If this was a general test for selection to the police as a whole I could see the argument. This was however a test for a specific role therefore the test should be absolute.
I am not a betting man but I would argue that the nature of the test (i.e. gender neutral) was influenced by the previous ruling above-not Custom and Practice.