There is no such thing as "defacto" legal tender. By definition only legal tender must be accepted as a form of payment - you can choose to accept something else but you don't have to.
I have no misconceptions here - I know Scottish bank notes are not technically legal tender, and thus, technically, nobody is under any compulsion to accept them.
But in the real world, they are absolutely defacto legal tender - as above, every bank in the UK (and even many beyond) accepts them.
I think the quarrel here is about the definition of "legal tender". Its literal meaning is not "reliably redeemable at a bank", but rather "required to be accepted by anyone as payment of a debt". These can diverge easily. For example, I remember that Sweden changed its banknotes a few years ago, but the old notes were redeemable at banks for a set period of time. The old notes were definitely not legal tender in the technical sense because private individuals and shops not only were not required to accept them, but actually did not accept them anymore. That doesn't work well with the definition of legal tender!
However, the old Swedish notes had not lost their value because they could still be redeemed.
In a formal legal tender situation, you could to some extent require a private party (within the relevant jurisdiction) to accept the notes in payment of a pre-existing debt. You could contrast this with "having value", "being redeemable", "being in circulation", and "being widely accepted", maybe.
Reminds me how bus drivers (and the like) won't accept highly denominated bills around here - while legally they ARE forced to accept them , but aren't required to give the change. (I haven't had the opportunity yet to see if I would be able to convince them that this is indeed the law.)
Literally every bank in the UK will accept Scottish bank notes - if that's not a defacto legal tender, I don't know what is.