"These arguments lead to at least three conclusions. 

The first is that Scotland does not have any obligation to compensate  England any part of the  so-called UK national debt as agent for the rest of the UK because no part of that debt can at present be attributed to Scotland.

The second conclusion is that even if Scotland did have a notional liability owing to the rest of the UK  the capital balance on the loan would not need repayment because there is no indication that the remaining UK will be repaying any of its debt, and therefore there is no reason for Scotland to make payment to the rest of the UK for something that they will not be doing.

Third, even if debt was due right now it would be the case that around £19bn was due to Scotland."
https://www.taxresearch.org.uk