This is a classic trap for the unwary question setter.
Mains voltage in the UK was traditionally 240 volts +/- 6%. On continental Europe, the standard was 220 volts.
The EU's decision of 1994, to 'harmonise' the standard European mains voltage at 230V, was a purely bureaucratic measure, intended to remove a perceived barrier to trade between European countries. There was no technical advantage in increasing the continental voltage or decreasing the UK voltage, and the cost of replacing the electricity supply equipment across Europe was prohibitive. So in fact, all that changed was the tolerances.
The UK regulations were changed (effective from 1 January 1995) to require 230 volts, with a higher tolerance of 10% and a lower tolerance of 6%. This allows anything between 216.2 and 253.0 volts. (Note that previously anything up to 254.4 volts was legal, so the increased upper tolerance of 10% almost makes up for the reduced standard voltage.)
To quote an answer on Yahoo (dated 2007): " ... [the UK electricity supply industry] keep[s] on generating as close as [it] can to 240V, the Continentals generate as close as they can to 220V, we all pretend it's 230V and everybody is happy!"
And another (also dated 2007): "It's a fudge and we all know it but it's probably one of the more sensible and pragmatic bits of standardisation that we've seen in the E.U."
It does, however, leave something of a dilemma for the quizzer who is asked "What is the standard mains voltage in the UK?" Arguably, the legal standard is 230 volts (with tolerances as stated in the previous paragraph); but the practical standard remains at 240 volts.
If you try and explain the above to the person who's asking the questions, I suspect that you may get short shrift. My advice, for what it's worth, is to answer "240 volts"; this was the expected answer the last time I was asked this question (in November 2014). If the question person responds with "Actually the regulations changed in 1995 ... " you have all the information you need in order to argue your case.
Good luck!
P.S. I am indebted for the above detailed information to TwoThirtyVolts.org.uk.
P.P.S. TwoThirtyVolts states that "In 2010, the voltage tolerance band may be increased to ± 10% but again, in reality, nothing will really change!" I can't find out whether this ever did happen (which suggests that it didn't) – but like they said: even if it did, the answer to the quiz question is still the same.
What will happen following Brexit is, for the time being, anyone's guess ...
Outside Europe, not surprisingly, each country has its own standards. The main thing to be aware of is that in the USA and Canada, and in many countries within their sphere of influence (several Caribbean and Central American countries, for example) it's 120 volts.
Japan has the lowest figure of all (unless there's one I've missed), at 100 volts.
In many countries with British heritage but not under the EU influence (e.g. Kenya, Kuwait, Nigeria and Uganda) it's still 240 volts.
Otherwise, 220 or 230 volts is pretty much a worldwide standard.
© Haydn Thompson 2017