I have never seen a cooker isolator tap on a gas cooker that you can assess in an emergency, however it is standard practice to put an isolator for an electric cooker within sight of the cooker but not above or behind it, as should there be a cooker fire the idea is you can still reach it, however as far I am am aware it is only common sense there is no rule to say it must be there.
Clearly no point if in the event of a fire you would not be able to assess it. Since you don't need to have one, there is no distance, just common sense.
In the flat under the house we do turn off the cooker isolator, but in the house I have never used the switch, the modern cooker has so many safety features not really required, I have found the hot plates turned on many times where wife has forgot to turn them off, but with no pan on them they turn off anyway and after a set time lock off, so no real problem, my only problem is if she does same in the caravan where we use gas.