[dfawcus]

Note that it is a "single currency", not a "common currency". In the same way that the "single market" is different from the prior "common market".

So if the UK had joined at the inception, it would have started as a common currency, but when the UK decided not to join, it reverted to the preferred form of some of the other members, namely the "single currency".

I imagine that the rules would have been different with a common currency, possibly with the national currencies not replaced, and still operating in tightly tracked bands; but I guess the UK fate there was written when it dropped out of the ERM as part of "Black Wednesday", caused by it entry peg being overvalued.

So a common currency may well have allowed for more north vs south flexibility...