@roger,
Sweden, Denmark and Norway already have kronor.
UK can keep their pounds.
Sweden, Denmark and Norway started kronor on the same level around 1890(?), but after about 30 years it did not work out any more.
If three countries so much alike could not have the same currency at the same exchange level how are 17 countries going to work out.
It took 30 years about 130 years ago when things moved so much slower.
I gave the Euro 10 years without problems.... sorry to say I was right.