The euro was officially launched on January 1, 2002, and is the official currency of the European Union, and is used in 19 of the 28 EU member countries, which are known as the Eurozone.
In 1997, the Chancellor Gordon Brown decided that the UK’s economy was not enough like that of other EU countries fro joining the euro to be a good idea. So Tony Blair’s government did not join the euro.
Since this decision was taken, many studies have been carried out and have come to varying conclusions; some have said that joining the euro would mean the UK could not have enough control over its own economy to be stable — an especially important point during the worldwide financial crash of 2009. Other studies said that if the UK joined the euro, we would trade more with the EU, and it would be cheaper as we would not need to exchange currency to do so — hence having a positive impact on the economy, though this was expected to have been minimal.
Legally, the UK is in an almost unique position regarding the euro. In 1992, John Major got an opt-out from the euro in the Maastricht Treaty, this means that the UK is one of only two EU countries (the other being Denmark) which will never be legally required to join the euro. For the UK to join the euro, there would need to be a change to the Maastricht Treaty, which must be agreed to by all 28 countries, whilst the other 27 are highly unlikely to block such a request from the UK — the UK would almost certainly block any attempt at forced change from the other member countries. Whilst it remains possible for the UK to join the euro, it could never be forced to join by legal means.