Applying eurail passes to a full fare ticket already bought and paid for

In theory yes, in practice this will give you so many headaches you´dd better not do it.

The main stumbleblock will be THEN to get seats on this €*-as for passholders there is only a smaller quota-and for coming weeks in busy summer already taken.

You MAY get money back for the sector in GB on LNER?-check conditions yourself, it will all depend on what type etc. Then you can start using a newly bought pass from LON to Scot. If that is still worthwhile fr how many other trips you intend to do