The Euro is a bit of a strange beast amongst currencies. When first introduced it was to a bunch of allied but separately managed economies and was treated, mistakenly, like those economies would continue to function exactly as they had before, just measuring their financial numbers using different units while the actual values involved remained unchanged. Of course, this structure was about as realistic as building a balloon out of bricks. Behind the jingoistic rhetoric spouted in public and the quieter, better-reasoned arguments about the penetrance of sterling in non-EU economies this concern was being quietly voiced in the UK and was a factor in the UK's declining to join in.
Economic policy within a nation can be broadly broken down into fiscal and monetary policy. Fiscal policy, the business of taxing to raise funds then spending them on running the government and funding government programs has a cycle of effects that are slow to emerge in an economy but which are recovered from comparatively quickly. Monetary policy, the business of interest rates and currency valuation, is the other way around. The effect of any change here is almost immediate but has a very "long tail." Typically a government needed its hands on "both levers" to manage an economy to balance short and long effectiveness times, long and short stabilization times after those effects were felt. The introduction of the Euro without a fully stabilized ECB already in place and the mechanisms for its interaction with member states governments already worked out took one of those levers out of the hands of every member government that joined the Euro. There were bound to be "issues" and some of them were bound to be severe. As the dice rolled, it was Greece and Portugal that took the major hits but it could just as easily have been France or any other major EU economy if the trigger factors had been different.
The member states and the ECB did learn from this, and the system is now much more stable. At this point many of the valid reasons - aside from dumb nationalism, that is - that the UK had for not participating in the Euro have gone away. There would no longer be a serious disadvantage to the UK from adopting the currency. Indeed, my personal opinion is that the UK should now join the Euro, whether we remain by revoking A50 or whether we stumble into leaving and later have to rejoin, either as a union or as individual nations. But selling that to the toxic "little Englanders" aint going to happen any time soon.