From the University of Nottingham, similar information from Schneider too. Dear old EU harmonisation has failed to actually make any significant difference as we still generate in UK at 242v average. Its only in law that we are 230v!
""Voltage optimisation
Voltage optimisation is a term commonly used to refer to the energy-saving technique of reducing the electricity voltage supplied to a site, in order to reduce losses in the equipment, thereby reducing energy consumption, CO2 emissions and the electricity costs. The lifetime of the equipment is also extended, because it generally runs cooler at the reduced voltage, with a consequent reduction in maintenance costs.
Most electrical equipment is designed to operate with voltages within the standardised levels (with an allowance of 4% for losses within the installation), so many items of equipment are supplied with a higher voltage than is required for satisfactory operation.
Just as it was in 1960, the supply voltage to domestic properties in the UK is still nominally 240 V AC at 50 Hz, although the declared voltage in the UK is now 230V AC +10% to -6%. Historically the domestic voltage was 240 V +/-6% (and 415V 3phase), whilst continental Europe was 220V (380V 3 phase). Since 1995 the nominal voltage across Europe has been 230V (400V 3 phase).
In practice nothing has been changed in the UK. The first stage of European voltage harmonisation required a supply voltage of 230 V of -6% to +10%, i.e. be between 216.2 to 253V from a starting point of 240V +/-6%, i.e. be between 225.6 to 254.5V.
The second stage of European harmonisation was due to happen from 1st January 2003. This was 230V +/-10%, i.e. to be between 207 to 253V. The second stage has still not been applied, as of April 2013. So the situation in the UK regarding supply voltage, stated in law is that it should be 230V -6% +10%.
It is not unusual for the supply voltage to vary substantially throughout the day. The average voltage in the UK is around 242V. This means that in the majority of properties, electrical equipment is running at a voltage much higher than needed, resulting in excessive losses in many types of equipment. This is due, in a large part that due to the so called harmonisation of supply voltages in Europe all new low voltage appliances are designed to operate at 230V or even 220V. Therefore it can be seen that much equipment designed to be operated at 230V and 220V may be being supplied with a voltage above this if we consider that the UK average supply voltage is 242V, much higher than is needed.""