//If this impression is correct, can someone on here explain what is undesirable (bad) about the Euro as a currency compared with, say, the Pound ?//
Because the Pound is tailored to the UK’s needs and the BofE can manipulate interest rates and exchange rates according to the UK’s fortunes (or otherwise). The euro is used by 19 nations who are economically very disparate. The economic cycles of those nations do not coincide, their economies are poles apart (think Germany and Greece, for example) and, individual nations - particularly the peripheral ones - have no way of using currency and interest fluctuations as tools to control their economy. The euro was introduced in the hope that it would drive economic, fiscal and political union. That was a grave error. It should never have been introduced before fiscal and political union was in place. That is never likely to happen with the EU as it stands and the introduction of the euro was possibly the gravest political error (of a few committed by the EU) since WW2.
//Under what legislation could Scotland's using the pound be as its currency be denied?//
I don’t believe it could be denied (unless the UK makes it illegal to take its currency out of the country as do some nations, such as India and Cuba). Dozens of countries use the $US without any explicit permission. The question that the Scots need to consider is what will they buy their Pounds with?