Steve,
How does anyone on planet Earth actually KNOW that joining the euro would have been bad for this country?

Here is what Guardian says about us not joining,
"A jolly good thing too, of course. The euro has proved to be exactly the job-destroying, recession-creating, undemocratic monster the doubters always warned it would be. This was not the received wisdom on the left at the time, when to suggest that the euro would be supercharged monetarism, Thatcherism with knobs on, was deemed unseemly. People who liked the euro were civilised, supported the arts, went to Tuscany or the Dordogne for their holidays. People who didn't like the euro drove white vans decorated with the flag of St George.

Today, it is hard to find even the most fervent euro enthusiasts in the Liberal Democrat party arguing for UK membership of the single currency. Disillusionment with what was once called "the Project" is almost total in the face of grinding austerity, a double-dip recession that has already lasted 18 months and a jobless rate of 12.2% and rising."
https://www.theguardian.com/business/economics-blog/2013/jun/02/britain-euro-what-if-joined