Ask H&H: light meters

Q: I keep my horse on a large livery yard where the owner recently installed a light meter in the indoor school. We used to pay an additional pound on our livery bill for lights, but this new system was introduced as unfortunately people regularly left the lights on when the school was empty. The owner has advised that £1 will last for 30min.

My worry is that because there is no dial to tell when the money will run out and no warning before the lights go out, it could prove dangerous. Can you tell me if, health and safety wise, this is allowable?