In texts and real life you may find the UK domestic mains electricity voltage referred to as either 220 Volts, 230 Volts, 240 Volts or even 250 Volts AC (Alternating Current). But which is correct? Well, technically, ALL of them (and more) can be deemed as correct! Let me explain:
Mains voltage in the UK used to be 240V (±6%) in mainland Britain (that's between 225.6V & 254.4V). It was, reportedly, 220V in Northern Ireland.
In the early part of the 2000 decade, the European Union decided to harmonise the UK systems and the Continental 220V systems to bring them "all together within the EU". They decided for them all to be at 230V AC. It would have cost millions to change the infrastructure in each of the countries concerned, so it was decided that, from 1 January 2004, the mains electricity supply across the EU countries must be 230 Volts AC (+10% / -6%) at 50 Hz (±1%), which is a voltage anywhere between 216.2 and 253 Volts.
Physically nothing needed to be, nor had, changed:
Everybody was happy:
But, more importantly, NOTHING HAD CHANGED!
In 2008 things were altered again, slightly, and the domestic mains supply across Europe was to become 230 Volts (±10%), that works out to between 207V and 253V. Again, as every country was within the limits, NOTHING CHANGED (only the wording of the regulation)!
So, as you can see, the UK mains voltage should be referred to as 230 Volts but 220, 230, 240 & 250 Volts (the latter usually only in older documents/used by the more mature person) are often used interchangeably.
The actual supplied voltage can fluctuate slightly as the voltage will drop during periods of heavy demand and rise during periods of low demand but the generating companies do an excellent job of keeping it within the limits.
Thanks to Antony Watts who asked for the clarification on UK mains voltage as there were different voltages mentioned on different pages on this site, I didn't even think about it!