Here are some golden quotes from the time were were told not joining the Euro would be a disaster (hindsight is great):
"The head of one of the world's biggest car companies reignited the debate over the single currency yesterday with a warning for the government that its £1.5bn investment in the UK would be at risk if Britain stayed out of the euro." - Toyota, January 2000
"Nissan, the Japanese carmaker, said yesterday that further investment in the UK depends on whether Britain joins the euro." Carlos Ghosn, Cheif Executive of Nissan, 2002
"With Greece now trading in euros, few will mourn the death of the drachma" Financial Times, 2001
"...the euro will create a single market of transparent prices, reduced exchange rate risk and a pan-European capital market. Each of these will give a stimulus to competition and productivity growth across Europe." Adair Turner (former head of CBI), 2001
"Even if it [taking Britain into the euro] is unpopular, I will recommend it if it is the right thing to do." Tony Blair, June 2001
"...by keeping the pound, the UK misses out on much of the trade creation now occurring within its principal trading partner, the euro-zone." Members of Imperial College, Harvard University and London School of Economics, 2002
"The most important function of monetary policy is to provide stability..." Nick Clegg (in support of the Euro, not long before the Euro crisis occured)
"it is difficult to see how the UK can afford to remain outside the Euro economically. At present the public opinion in the UK is against joining the Euro, and for keeping the pound... Almost all change is resisted for various reasons, mainly psychological. Fear of the unknown, laziness in trying to avoid the extra the adaptation will require." Some bizarre reasons given to ditch the pound by a body promoting the Euro in the early 2000s.
"Britain will join the Euro, and we will start moving towards it during 2002. It is only a question of time how long we can afford to remain out of it. Sweden and Denmark also can hardly afford to remain apart from the Euro for more than a few years." Same group in the same year during the Euro hype.
Single currencies have never worked, throughout history...