Since London is a very big city, it may take a while for a plumber to reach your apartment, in case of a plumbing issue. While you wait for him or her, it is advisable to shut off the water valve, to prevent the problem from escalating. The water valve that will stop the flow of water in the whole system, is located either in the basement, or near the water meter outside your house. Call Safe Plumber, they will be at your home quickly.