And for Scotland to be a formal part of such a sterling zone, the Bank of England and HMTreasury would wish to impose constraints on the financial risks that Scotland's banks could take - to limit liabilities for British taxpayers.
Finally, it is worth remembering that the Bank of England and HMTreasury have announced two new schemes to provide copious amounts of cheap loans to British banks, which can be seen as insurance against the downgrades leading to any kind of renewed credit crunch (see this blog for more on all this).