The national average supply of our voltage in the UK is around 240 volts but this can rise and fall due to fluctuations in supply and demand. This can be caused quite simply by lots of people in the area using more electricity at once, ie cooking dinner in the evening or, if there are local factories using large electrical equipment that is turned on and off. In theory, a supply that should be 230 volts could be fluctuating as much as 10%, giving you an actual voltage anywhere between 210-250 volts.
The voltage we need in our homes varies across different electrical equipment. The electrical motor in a fridge or a vacuum cleaner require a higher voltage to force the current through, where as a light or laptop require considerably less. Essentially, because the voltage coming in is often more than required, the energy is being wasted, costing you money as well as reducing the life of the equipment.