Retro - an isolator is just a switch that they fit between the incoming main and the meter. It makes it easier and safer when working on consumer units. It means you can just switch off the power.
It's about time suppliers provided this. Otherwise, you have to pull the supply cut-out (big 100 amp fuse) to switch things off. Technically, you shouldn't do this as it means cutting a wire seal, but what else can you do?