“there is no purpose to law without sanctions, and this one appears to have none beyond the extra capacity for embarrassment if it is broken.”

First, I suppose it’s hoped by the charter’s authors that successfully adhering to it will lower the cost of borrowing, other things equal.  So the sanction is higher taxes [fewer votes, more restless right wing MPs] for the same amount of public spending.

I’m not so sure the Charter can or will be adhered to, and whether trying might not lead to more uncertainty, rather than less, but leave that aside for the moment.

Second, the reputational costs Jeremy mentions are not to be sniffed at.  These seem to have served us reasonably well in the case of the legislation isolating the Bank of England’s monetary policy decisions from political interference.