Delays in getting a response can lead to delays in a person being able to take up an appointment or office to the detriment of the individual and the body concerned. Once a check has been made, it is good protection for the "employing" body and should be for any other. A single check saves money and resourcesm and should lead to fewer volunteers being put off.
Like a TV or vehicle licemce, a SRB check should be valid for a fixed period and acceptable to all bodies requiring it, avoiding the need to get a new one for every new appointment where one is needed.